Google updated both its image SEO best practices and Google Discover help documents to clarify that Google uses both schema.org markup and the og:image meta tag as sources when determining image thumbnails in Google Search and Discover.
Image SEO best practices. Google added a new section to the image SEO best practices help document named Specify a preferred image with metadata. In that section, Google wrote:
“Google’s selection of an image preview is completely automated and takes into account a number of different sources to select which image on a given page is shown on Google (for example, a text result image or the preview image in Discover).”
Here is how you influence the thumbnails Google chooses:
Specify the schema.org primaryImageOfPage property with a URL or ImageObject.
Or specify an image URL or ImageObject property and attach it to the main entity (using the schema.org mainEntity or mainEntityOfPage properties)
Specify the og:image meta tag.
Here are the overall best practices when choosing these methods:
Choose an image that’s relevant and representative of the page.
Avoid using a generic image (for example, your site logo) or an image with text in the schema.org markup or og:image meta tag.
Avoid using an image with an extreme aspect ratio (such as images that are too narrow or overly wide).
Use a high resolution, if possible.
Google Discover image selection. In the Discover documentation Google added a section that reads:
“Include compelling, high-quality images in your content that are relevant, especially large images that are more likely to generate visits from Discover. We recommend using images that meet the following specifications: At least 1200 px wide, High resolution (at least 300K) and 16×9 aspect ratio”
“Google tries to automatically crop the image for use in Discover. If you choose to crop your images yourself, be sure your images are well-cropped and positioned for landscape usage, and avoid automatically applying an aspect ratio. For example, if you crop a vertical image into 16×9 aspect ratio, be sure the important details are included in the cropped version that you specify in the og:image meta tag).”
“Enabled by the max-image-preview:large setting, or by using AMP“
“Use either schema.org markup or the og:image meta tag to specify a large image that’s relevant and representative of the web page, as this can influence which image is chosen as the thumbnail in Discover. Learn more about how to specify your preferred image. Avoid using generic images (for example, your site logo) in the schema.org markup or og:image meta tag. Avoid using images with text in the schema.org markup or og:image meta tag.”
Why we care. Images can have a big impact on click-through rates from both Google Search and Google Discover. Here, Google is telling us ways we can encourage Google to select a specific image for that thumbnail. So review these help documents and see if any of this can help you with the images Google selects in Search and Discover.