Being original with a meme is crucial. You must work your own memes. I sometimes, preparing my memes with days in advance for that contest...

Yes some old memes copied from random posting are also good, but depend on which moment you post them.

I can understand when somebody copy a meme and post it as a reply or something like that. That is just fine. But copying a worked meme and then post it in a contest is quite lame. At least mention the credits goes to x.