Its pretty common. Deviant art doesn't exactly do a great job of letting people know that the stuff there is copyrighted. That info is usually tucked into a corner somewhere

At any rate, just make sure the new art you use is under an open use license or you have permission from the author to use it.
Its kind of a pain, but it is very VERY important if the art is to ever actually make it into the game.
Sxc.hu and wikipedia are usually safe bets since everything on those sites is generally under some kind of open use license.
Happy hunting.
Spoiler for In case you have trouble finding viable art:
If you are having trouble finding good free to use art, you can use this if you want

just credit the original source from sxc.hu:
-Original Image by:Geogre at en.wikipedia (Later version(s) were uploaded by Drat at en.wikipedia.)
-Modified by OdinVanguard using GIMP