I don't know about dedicated ones, but I would explore here:

Flickr: https://www.flickr.com/search/advanced/ (turn on CC image search)

http://creative-commons.deviantart.com/

http://source.pixite.co/

http://allthefreestock.com/

Desktoppr is a nice wallpaper app that puts images in your dropbox. A lot of fantasy and sf art in the mix but not exclusively

Are you thinking of building something? I'd say there's a market for it (though no obvious monetization strategy I can think of)

There are tons of sites that let you download tons of 3D models for free. The problem is that the majority of those models were ripped off out of commercial games and projects and there is no hint of a valid Creative Commons or any other kind of license to use those items.

http://deviantart.com

#payyourartists !

There's artstation, you could look around and commission/buy rights to images from there.