Our website mainly consists of galleries, while usually smaller, they sometimes reach over 100 images per post. As each image is bound to the same topic, they are automatically named after the post title and an added incremental number at the end, for example:
League of Legends Sona Cosplay By Eki
league-of-legends-sona-cosplay-by-eki-1.jpg league-of-legends-sona-cosplay-by-eki-2.jpg league-of-legends-sona-cosplay-by-eki-3.jpg
At first, all of those had the page title set as their alt text and everything was fine for a long time like that, but then I got scared that Google might find that keyword stuffing or something, so I removed all the alt tags and they are now alt="". This seems like a really bad practice, so I was wondering what could I do to fix this and I came up with an idea to simply use the file name as an alt tag, but this still leaves a lot of duplicate content issues if you ask me.
I am not trying to trick Google into ranking better, but merely trying to protect the site from any penalties that could arise from all the images there.