On Twitter, Google’s John Mueller confirmed that a website can be ranked for words that are used in the img alt attributes of the web pages.
Pages can rank in websearch for content that's in image alt-attributes, so I guess that's a yes :). Easy to try out, if you want to double-check.— 🍌 John 🍌 (@JohnMu) June 19, 2019
Folks have been stuffing alt attributes (or alt tags as we used to call them) for a really long time now. Our algorithms have learned to interpret them appropriately :-).— 🍌 John 🍌 (@JohnMu) June 19, 2019
How to check the alt attributes of your website images
Among many other things, the website audit tool in SEOprofiler checks the img alt attributes on your web pages. The Top 10 Optimizer tool in SEOprofiler also analyzes your pages and it shows you how to improve the img alt attributes (and other things) to get higher rankings: