Ms. Cat, for an artwork to be 'original', you have to be able to claim authorship over ALL of the work, not just parts, or the majority of it.
A perfect example is all of the Community Skin projects or the one at Deviant, as well. I did LiteSTEP themes for all three, but the graphics and walls were either directly or indirectly from others in the 'collective', and were not 'specifically or completely' mine.
The graphics in these cases were released for the express purpose of creating the suite of skins, and so we all involved had exclusive rights to use them.
Of course, if I were now to use those graphics in a different project of my own, the permission I had to use them for the CS project would NOT follow on to this latter one.
I feel my brain wandering...so I'll stop...but you probably get what I mean....