I see many imageboard sites use keywords in the image alt. I read articles that keyword stuffing on your image's alt will cause you to rank bad. But I see many imagboard sites still do it but still rank well. An example imageboard site: http://safebooru.org/index.php?page=post&s=list they put lots of keywords in their image alt and still rank well. How do they do it?
You must have also read that SEO alone cannot make a site rank well. Its your viewership, and viewership will depend on good content. People might love to see those anime pics, therefore they visit that site.