I have noticed that there are quite a few wallpapers being posted, especially in the iPhone 5 (Shelves) section that are not original creations.
I'm not naive enough to believe that folk can be prevented from copy-cat uploading. Some believe that it's perfectly ok to do so and they're only 'sharing' files.
That's a valid argument I suppose but only if it's made clear that the contributor is not the author.
I've started watermarking my creations again as a result.
It would be nice if contributions to M9 by original artists could be clearly identified as such, and copied contributions similarly marked.