Here's a random one I just whipped up.
I think this is a great idea, because you can make your poster say just the right thing.
AND it could be used to create posters that have nothing to do with motivation... obviously your caption can say anything you want it to. (Mine is borderline motivational, I guess.)
For example, I think it could be a great way to create visual aids that demonstrate a concept, like Juxtaposition.
So, should you feel inclined, here is something fun you can waste a few minutes on. You're welcome.