If you're a decent human being, take down all of the work in your gallery. The original owners worked hard to make these art pieces to have someone like you reposting them
The least they could do is credit the owner. If they don't know who the creator is and won't go through the trouble on looking, they might as well do everyone a favor and not post it at all. They're not being of any help to the original artist by posting their work unsourced....