Interesting topic.

I've always felt that art is meant to be shared, properly credited, but shared. If someone's work sync's up with another's imagination, or better, inspire a character outright, I think using an image is entirely appropriate. Practically speaking though, with work's produced for and posted on the Internet to receive feedback/be displayed, it is difficult to enforce permission mostly because you can't be certain the person posting is actually the one who created the work anyway.

I do hope if an artist would ask for the work to be removed that it would be respected and removed.