If you are drawing  by hand (or by computer, etc.) a character or scene based on a famous character or scene, if it is recognizable as coming from the source material, it is most likely a copyright violation; that is because copyright bars people from creating "derivative" works, or works based on, a copyrighted work. To use a comic book example: a new original hand-drawing of Marvel's "Thor"--i.e. of a blond god of thunder, wearing a similar outfit, and holding a short-handled, large-headed warhammer--would be a copyright violation, even if it is a fully new, original work. To sell a new "Thor" shirt, you'd have to go back to the original (uncopyrighted) source material (the Norse myths) and come up with your own "Thor" design that does not look anything like the Marvel Comics verions (e.g. red haired; make the warhammer look like a historical warhammer, not like a block of stone on a stick; etc.). So if it's recogizably based on someone else's character or scene, you could be violating copyright (and, in many cases, especially for more famous characters, trademark, too).Yes, you do see shirts with copyrighted characters on them--unless the manufacturer licensed the right to use the character, they are in violation and they are just counting on the rights owner not noticing them or choosing to take action.