How Google Picks Images for Search and Discover (And Why Most SME Sites Get It Wrong)

You spent time finding a great image for your blog post. You uploaded it. You published the article. Then you searched for it on Google and the thumbnail showing up was... your company logo.

Or worse: a random sidebar graphic. A stock photo from three pages down. A product image that has nothing to do with the article.

This is not a random occurrence. Google selects images automatically, and it follows specific rules to do so. Most SME websites were not built with those rules in mind, which means Google regularly overrides the image you intended to use with whatever it decides is most relevant.

Key Takeaway: Google selects thumbnails for search results and Discover feeds automatically based on image resolution, markup signals (og:image or schema.org), and relevance to page content. To control which image appears, you need to specify it explicitly in your metadata. That image must meet Google's technical requirements (particularly for Discover, where 1,200px+ width and a 16:9 ratio are recommended). Sites that rely on logos, stock photos, or CSS background images are likely losing clicks because Google is overriding their visual with something less compelling.

Written by Derek Chua, digital marketing consultant and founder of Magnified Technologies. He has helped Singapore SMEs improve organic search performance through technical and content SEO.

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Why Your Thumbnail Is a Conversion Decision, Not Just a Design Choice

When a searcher sees your result on Google, they process three things almost simultaneously: your title, your description, and your image. For standard blue-link results, the image appears as a small thumbnail beside the snippet. For Google Discover (the feed Google shows on Android and iOS based on your interests), the image dominates the card. A low-resolution image, a logo, or an irrelevant graphic does not inspire a click.

Google has published documentation explaining exactly what signals it uses to select images. Most websites, especially smaller ones that were built without this knowledge, are unintentionally handing Google the wrong signals, and losing clicks because of it.

How Google Actually Decides Which Image to Show

Google's image selection process is fully automated. It crawls your page, identifies all images present, evaluates several signals, and makes a decision. You can influence that decision, but only through the right channels.

The signals Google considers include:

Explicit metadata signals (highest weight):

• The og:image Open Graph meta tag
• The schema.org primaryImageOfPage property (linked via JSON-LD)
• The image property attached to the main entity in structured data (e.g., BlogPosting)

Page-level signals:

• The image's position on the page relative to the main content
• The alt text and surrounding text context
• Whether the image is embedded as an HTML <img> element (CSS background images are not indexed)
• The filename (descriptive filenames provide light signals to Google)

Image quality signals:

• Resolution and dimensions
• Relevance to the topic of the page

The most important thing to understand: if you do not provide an explicit metadata signal, Google will make its own choice. And Google does not always choose what you would expect.

The Discover Requirements That Most SME Sites Ignore

Google Discover is different from standard Search. It is a recommendation feed. Google decides what content to surface based on a user's interest profile, not a specific search query. Getting into Discover can drive significant traffic, but Discover has stricter image requirements than standard search:

For a large Discover thumbnail (the kind that generates clicks), Google recommends:

• At least 1,200px wide
• High resolution (at least 300K file size before compression)
• 16:9 aspect ratio
• The max-image-preview:large robots meta tag enabled (or AMP)

Many SME websites were designed with images sized at 600–800px. These are adequate for the web design, but inadequate for Discover. Google may still show these pages in Discover, but with a small thumbnail rather than the large format card that generates substantially higher click-through rates.

Additionally, Google explicitly states: avoid using your site logo or images with embedded text as the og:image or schema.org markup value. These generic images reduce your chance of a compelling Discover card even if everything else is correct.

The 5 Most Common SME Image Mistakes

These are the patterns we see most often when auditing SME websites:

1. No og:image tag at all. Google makes its own choice. The result is often a sidebar image, an author headshot, or a small icon. None of these represent the article.

2. Using a company logo as the og:image. It matches the brand, but it tells the reader nothing about the content. Google's own guidelines say to avoid generic images like logos in your og:image tag.

3. Images sized at 800px or below. These will not qualify for large Discover thumbnails. You may appear in Discover, but as a small-card result rather than a large visual one.

4. Images embedded via CSS background-image. Google does not index CSS images. If your hero image is set as a background in CSS, it will never be considered as a thumbnail candidate.

5. Images with embedded text. Infographics or promotional images with copy on them are specifically discouraged by Google for use in og:image and schema.org markup. They may render poorly at thumbnail size and Google may override them.

How to Specify Your Preferred Image (The Practical Way)

There are two reliable methods to tell Google which image you want displayed:

Method 1: og:image meta tag (fast, widely supported)

Add this to the <head> of your page:

<meta property="og:image" content="https://yourdomain.com/images/your-article-image.jpg">
<meta property="og:image:width" content="1200">
<meta property="og:image:height" content="675">

If you are using WordPress, the Yoast SEO or RankMath plugins generate this automatically from the featured image. The critical step is actually setting a featured image on every post and making sure that image is at least 1,200px wide.

Method 2: schema.org structured data (more authoritative signal)

In your JSON-LD structured data block:

{
  "@context": "https://schema.org",
  "@type": "BlogPosting",
  "mainEntityOfPage": "https://yourdomain.com/your-article-url",
  "image": "https://yourdomain.com/images/your-article-image.jpg"
}

This method provides a stronger, more explicit signal than og:image alone. For Discover eligibility, combining both is the strongest approach.

For Discover specifically: Also add this to your robots meta tag or equivalent:

<meta name="robots" content="max-image-preview:large">

This tells Google it is allowed to display large preview images from your site in Search and Discover. Without it, Google may restrict itself to smaller thumbnails even if your image qualifies in every other way.

A 6-Step Image SEO Audit for SME Websites

Run through this checklist for your five highest-traffic blog posts or landing pages first. Then roll it out site-wide:

1. Check your og:image tags. Use a tool like Facebook's Sharing Debugger or your browser's developer tools to confirm the og:image tag is present, and that the URL it points to is live.

2. Measure your image dimensions. Right-click the image on the page, open it in a new tab, and check the URL for the actual resolution. If it is below 1,200px wide, resize and re-upload.

3. Confirm aspect ratio. A 1,200 x 675px image gives you a standard 16:9 ratio. If your images are square or portrait-oriented, they are less likely to qualify for Discover's large card format.

4. Audit your alt text. Every <img> element should have a descriptive, relevant alt attribute. Not keywords stuffed in. Write a sentence that describes what the image shows and why it is relevant to the content.

5. Replace logos and stock imagery in og:image. If any of your pages use a logo or an irrelevant stock photo as the og:image, replace them with a content-specific image.

6. Check your robots meta tag. Search your site's <head> for max-image-preview. If it is absent or set to none, update it to large.

What We See at Magnified

When we conduct technical SEO audits for clients, image metadata issues are almost universally present. The most common scenario: a business has invested in good photography or design with real images that represent their services, but none of those images are connected to the right metadata signals. Google is choosing thumbnails from icons, sidebar widgets, or unrelated promotional banners.

Fixing this is not a complex undertaking. Most of it can be done in a CMS in an afternoon. But the impact on click-through rate from both Search and Discover can be meaningful, particularly for businesses that are generating Discover traffic already without realising it.

The first step is knowing the rules exist. Most SME websites were built by designers who optimised for how the site looks in a browser, not how it appears as a thumbnail in a Google feed. Those are two different problems.

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Frequently Asked Questions

Does Google always use the og:image I specify? Not always. Google describes its image selection as "completely automated" and says it takes into account "a number of different sources." The og:image tag and schema.org markup are the strongest signals you can provide, but Google may override them if it determines another image is more relevant to a particular query or feed context. The best approach is to provide high-quality, content-specific images through both channels.

What if I do not have any images at all on a page? Google may still index and rank the page, but it will not receive a thumbnail in search results or Discover. For pages where you want visual presence in search or Discover, adding at least one relevant, high-quality image is worth the effort.

Does image file size matter, or just dimensions? Both. Google's Discover documentation references a minimum of 300K as a high-resolution threshold. That said, oversized images slow down page load speeds, which affects ranking. The target is high resolution with proper compression. Google's PageSpeed Insights will flag images that are excessively large relative to their display size.

My WordPress site uses Yoast. Am I covered? Yoast automatically generates an og:image tag from your featured image. But this only works if you actually set a featured image on each post, and if that featured image meets the size requirements. If you rely on Yoast without setting featured images, Yoast may fall back to your site logo, which is exactly what Google's guidelines say to avoid.

Does image SEO affect regular Search rankings, or just thumbnails? Both. Google uses alt text and surrounding image context as ranking signals, not just thumbnail signals. Descriptive, relevant alt text contributes to how Google understands your page's topic. This has a direct (if modest) effect on organic search rankings, not just how your result looks.

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If your website is generating organic traffic but your click-through rate feels lower than it should be, image metadata is one of the first things worth auditing. It is a technical fix with a visual outcome, and it is one most SME sites have never looked at.

Contact Magnified for a technical SEO audit.