Img Alt Text is Used for Both Web and Image Search
Google treats the text in an image alt attribute as part of the page for web search, as well as information about the image itself, for image search.
Image Alt Text Should Only Describe What’s Shown in the Image
Image alt text should focus on what’s shown in the image and not include other associated keywords which are not relevant to the image.
The URL Inspection Tool Can’t be Used to Test Images
Google needs to be able to find clean image tags pointing to the image URL with a proper source attribute, which can be affected by lazy loading and responsive image loading based a device. You can’t test images directly using the URL inspection tool, but you can check the rendered HTML to see if the image links are being recognised.
Keep One Version For Image File Names Rather Than Having Multiple Translated Versions
John recommends against creating translated file names for images and instead keeping one version. Image file name is used as a small ranking signal, but the text on the page it appears on is a much stronger signal.
Implement Material Design Icons Using a Code Point Rather Than Text Element
If you are implementing material design icons using the i text element with a specific class, Google will see this as text on the page and will try to index and display it within search results. John recommends specifying these icons using a material design code point, which Google will read as a character, rather than text.
Use Alt Tags as Anchor Text when Linking with Images
When using links within images, make sure the alt attribute is being used in order to provide the anchor text that you want associated with the link.
Embedding Images From Other Sources Can Impact Image Search Traffic
Embedding images from other sources via iframes makes it more difficult to index an image alongside a particular page as opposed to an image embedded normally in the HTML. On top of that, sources like Instagram add noimageindex within the embed code which will prevent images from being indexed.
Use the Mobile Friendly Test to Check the Implementation of Lazy Loading
If you are using JSON or JQuery to retrieve images for lazy loading, using the mobile friendly test will allow you to see if these are able to be successfully loaded or not.
When Optimising For Image Search Focus on The User Journey
Rather than just optimising images technically, John recommends focusing more on how a user will search for the visual content on your site.