Ideally that could be classified as a derivative work, which one could argue is fair use. But, that argument will happen in a court after you have been sued. The "Kind of Bloop" affair should give you an example of what I'm talking about: http://waxy.org/2011/06/kind_of_screwed/

Honestly though this is just couch-analytics from me. I'm not sure how copyright applies to meme pictures and am fascinated by related court cases. Anyone else have interesting meme/copyright-related court case suggestions?