Well, on any html/css page pictures can have alt text. I do that here on Wikitree when I use the [[image]] format, rather than the {{image}} format. (The second can have a caption, but it usually just names the people in the image, it doesn't describe it.)
I have two nephews who are legally blind. They use screen readers. Because of them, and others like them over the years, I have tried (I don't always manage it) to remember to add a text explanation if I post an image on, say, facebook. It's why I use the alt-text here, for mouseover. (example can be seen here: https://www.wikitree.com/wiki/Alderdice-63 )
The flag images in stickers, for example, just tell you the image name, not what it depicts.
Is your project going to encourage more people to add proper descriptions to images, or to use alt text? If so, how? (People aren't always aware, or they don't remember, or a myriad of reasons they post a picture and no explanation. And g2g isn't good for alt text on images.)