Thenkan is right. Actually, Motivation Posters as a stile of humor are build on using another work and comment it. It is all just a comet of concert picture in general.
Of course, once I know I am using work - made picture of somebody else from DeviantArt, I´ll post his name if I mange to found it out. And of course, I would delete or change my poster If author of original picture disagree with posting it.
However, charge motivation poster as steeling is same as if you blame for steeling people who post screanshots from tv shows, movies and games, or even people who d
Also, by laws in most of countries, making fan pictures, even if you draw it for your self is direct breaking of copyrighted work.
Everyone who is, and most of people are aware of these facts understand that I taking credit, and present us my work ONLY comments, not a pictures itself, and I AM NOT using my motivation posters to present/attack any group, neither making many or present another people work us my own.