There is a difference here that I think is being missed. It is not necessary to give credit for everything in the image, whether you originally made it or not. It is only a courtesy (unless it is required via a license agreement). And what seems to be missed is that if you DON'T give credit, some are assuming that means you are claiming you made the stuff. And that's simply not true. You can claim copyright on your image, whether you made all the stuff that shows in it or not. (As long as you have the right to use the stuff.) The finished render IS your work legally. Philosophically may be another story ;-)