Flickr, dA, and Google Image Search all have an option to only search creative commons images (and either limit it to noAttribs CC or pass along the attribution in the API. At least in theory. I couldn't figure out how to get to it in dA's API.) (and I certainly wouldn't try it with a fan-run fanart archive unless it had some kind of CC option.)

Tumblr's would theoretically use tumblr's tag search, which only searches OPs. It doesn't have a CC option, but tumblr's TOS includes permission for images to go out on the API, and since wrotegoat wouldn't do anything except dynamically hotlink with a link to the OP, it seemed mostly ok? But I didn't even get as far as looking at Tumblr's API.

Pinterest is a mess.