Google Explains How It Selects Thumbnails for Search and Discover

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Google Updates Image SEO and Discover Guidelines to Specify Preferred Images

Google has added guidance to its Image SEO and Discover documentation, explaining how publishers can indicate which image should appear as a thumbnail in search results.

The new section, “Specify a preferred image with metadata,” highlights two key ways to influence image selection:

  1. Schema markup:

    • Use the primaryImageOfPage property on a WebPage type.

    • Attach an image property to the page’s main entity via mainEntity or mainEntityOfPage.

  2. Open Graph meta tag:

    • Include the og:image meta tag in the HTML <head> to indicate the preferred image.

The documentation includes code examples for each method, showing how to implement structured data in JSON-LD blocks or standard meta tags.

Best Practices for Preferred Images

Google also provides recommendations to ensure thumbnails are effective:

  • Select images that are relevant and representative of the page content.

  • Avoid generic images, such as site logos or images with embedded text in metadata.

  • Avoid images that are too narrow or excessively wide.

  • Use high-resolution images whenever possible.

According to Google, these updates were made “based on feedback” to clarify that both schema markup and og:image are considered when selecting images for Google Search and Discover thumbnails.

Google Updates Discover Documentation on Image Thumbnails

Google has updated its Discover documentation to clarify how publishers can influence which image appears as a thumbnail. The update links to the new Image SEO section and emphasizes the use of schema.org markup or the og:image meta tag to specify a preferred large image.

Key Points from the Update

  • Eligibility for Large Images: Large previews in Discover still require the max-image-preview:large setting or AMP. Metadata alone cannot bypass these requirements but can help signal which image to prioritize.

  • Avoid Generic or Text-Heavy Images: The guidance reiterates that images used in schema.org markup or og:image tags should be representative of the content, not logos or images with heavy text.

  • Connects Metadata to Existing Recommendations: Previously, Discover guidance included general image quality advice. The update explicitly ties those recommendations to the metadata methods.

Why This Matters

The updated documentation clearly identifies two ways to signal a preferred image:

  1. Schema.org structured data (primaryImageOfPage, mainEntity, mainEntityOfPage)

  2. og:image meta tag in the HTML <head>

Google’s existing recommendations remain:

  • Images at least 1,200 pixels wide

  • High resolution (minimum 300 KB)

  • 16:9 aspect ratio

  • Large images increase the likelihood of clicks from Discover

The new metadata section explains how to provide a preferred image that meets these specs, giving publishers more control over which thumbnail Google selects.

Looking Ahead

This update clarifies existing practices rather than changing how Discover works. Publishers can review the new section to ensure their markup aligns with Google’s guidance, helping improve thumbnail selection for Search and Discover.