Wolfram Alpha has the best definition:
http://www.wolframalpha.com/input/?i=meme
A term coined by Richard Dawkins in his book The Selfish Gene that was originally used to describe packets of cultural information, but subsequently adopted by internet users to refer to inside jokes for people who have no friends with whom to have real inside jokes.
So yeah, a meme isn't just a funny picture of anything, lolcats would be a meme, or the xzibit thing, stuff like that.