Is there any way to import pictures into Forza 5. Because there are some cars that almost look impossible to build using Fh5 Editor?

No, you can’t. Never have, never will. It’s all done with those shapes. See this thread.

Actually, there is now a way to create images in the vinyl editor using images or photos scanned outside the game. I am not going to discuss it any further here as it’s not a legitimate method and does break Forza’s TOS, but it is possible and its use is spreading. So it’s very possible that some of the images referenced above are actually being created outside the game.