Yes, according to official statements from Google (quoted below), the alt text is a ranking signal.
“anchor text (and image alt text) helps us quite a bit in understanding context, so I wouldn’t leave it out if you can avoid it.” source
Yes, but anchor text (and image alt text) helps us quite a bit in understanding context, so I wouldn't leave it out if you can avoid it.
— 🍌 John 🍌 (@JohnMu) May 15, 2017
“Alt text is extremely helpful for Google Images — if you want your images to rank there” source
Alt text is extremely helpful for Google Images — if you want your images to rank there. Even if you use lazy-loading, you know which image will be loaded, so get that information in there as early as possible & test what it renders as.
— 🍌 John 🍌 (@JohnMu) September 4, 2018
Q: “Does the alt image tag play a role in rankings for non-image search? Should we be putting descriptive comments on what the pictures are to help with general search results”?
A: “So, the text in the alt attribute for image tags is seen as a part of the text on the page. So, that’s something where it does help us a little bit to understand that page better[…]. It does play a big role for image search though. So, if you have images that you want to have appear in Google images, then make sure that you have a clean alt text there. Also, make sure that you watch out for the other aspects that relate to image search, as well.” source