checking pic plagiarism isnt easy, but image search does ease things a bit.
also, if a person claims an exotic flower pic, a google search using as close a name as possible could bring out the location and sites that those sort of images were lifted from.
apart from that, one could just go around verifying the identity of the person claiming a pic, and cross ref that with their steemit posts, and their previous posts etc.
It is a really hectic job. But I think any length should be reached to bring defaulters to book. I have come to understand that picture plagiarism is very very bad