Google Uses Alt Attribute as Part of Anchor For Linked Images
For linked images, Google sees the alt attribute as a descriptive part of the page it is on, but it is also used as part of the anchor for the page the image is linking to.
Alt Text Helps Google Understand Pages Beyond Image Search
Text in the alt attribute of images is seen as text on the page and can help Google better understand the page a little bit better beyond image search. However, the text in the alt attribute will likely be somewhere on the page anyway.
Check Alt Text is Present on Mobile Version of Images
John recommends checking that the alt text is present for the mobile version of images for mobile-first indexing. This is something Google has seen missing from sites that serve different content on mobile.
Data-alt Attributes Have No Impact on Image Search or Regular Search
With regard to images, data-alt attributes are used more for lazy-loading and giving other information for images from scripts, but this isn’t used for indexing in any way.
Linked Image Alt Text Acts as Anchor Text
Add alt text to image links as these act as anchor text, including on mobile pages.
Alt Text is Included as Part of the On-page Text
Alt text is shown when images are turned off in a browser and counts as part of the on-page text.
Add Alt Text and Embed Images on Mobile Sites in Preparation for Mobile-First
Google’s Mobile-first team has noticed many sites don’t include alt text for images on mobile sites. John recommends checking that alt text exists for images when checking sites are ready for the switch to Mobile-first indexing as well as making sure images are embedded so they can still appear in Image search.
Add Images to Sitemap to Provide Google With More Information
Images can be added for each page of your site within your sitemap along with captions and alt-tags using special sitemap extensions.
Image Links Pass the Same Weight as Text Links
Provided you have alt text on the image, which is equivalent to the anchor text, image links should be treated the same as text links.