As I understand it, the protocol is as follows:
1) If it's your own work, you don't really need to say so. It's implied by virtue of not being specified.
2) It it's someone's image, give credit as far as you know credit is due. If you don't know who to credit, say so up front and odds are someone will point you the right way. If not, at least you aren't accidently claiming it as yours by not specifying it isn't.
3) If it's your own render of someone else's mesh, say so and credit the mesh. Even if it's a mesh that you have modified even in just some minor cosmetic way.
If you're cribbing stuff taken from screen caps of the show, that's cool, I think we'll all understand where that came from, but specifying that it did come from the show certainly wouldn't dig you any deeper.
I hope this clears up any misunderstanding for the future! I can see how it could be easily overlooked if you're new at this game.