It says you can 'find or create' a meme. I think the Guidelines section is copy-pasted from a straight Paint contest, so the part about the Paint look probably does not apply. Similarly, it's practically impossible not to plagiarize on some level: all the original ideas have already been used. By Shakespeare.
So I think found memes are fine. Modified memes might be better. Memes you make up yourself? Go for it!
I do think, though, that one or two people posted a meme that was already used, and one person posted multiple memes (I think the judges will take the first into account and discard the rest so as not to unfairly disqualify the entrant). Please check your work to make sure everything's good.
If English is not your first language, you also may want to be sure your English is gamer-English, if not proper English, so it can be easily understood. Have a friend check it instead of relying on Google Translate, which is not always accurate.
And of course, have fun.
Support your friends; share your spares.