Well, doujinshi is also commercial and for profit, while fanart is more 'for fun' yanno? Sometimes fanart and doujinshi somewhat converge, so I try not to use too much doujinshi in general.

and if you wanna be copyright pedantic, you could say that you should own official art or concept art and it'd go to the point where you're like, NO YOU CAN'T USE THAT PICTURE OF A TREE YOU TOOK YOURSELF, DID YOU MAKE THAT TREE? COPYRIGHT INFRINGEMENT, BITCH, GOD'S A=SUING YOUR ASS.

Anyways, tl;dr, will get to it ~