You may only use art for cards that have the text "+1/+1 counter" on them. The medium is a banner. Your banner should have more to do with the counters than just having art of cards that mention them, but you won't be disqualified if it doesn't.
I'm hardly a pro. Feste's banner looks like he spent a lot more time on it, and we're all still waiting for Topher and the Togs to come in and blow us all away.
Advice: don't withhold submission because you feel you have no chance. Just do your best and submit that. My first couple entries were all but laughed out of the contest, but I stuck with it and learned new tricks and techniques. You, too, will eventually learn how to do stuff that you can't imagine doing right now, and then you'll start vying for Fogatog's spot atop the leaderboard (hopefully not too hard, though; I want it too ;):D).
Funny how so far 3 have to do with counters and 7 with cards that have counters...
I would count Darklight's as dealing with counters. He just decided to do something besides the typical bead. And others have to deal with the +1/+1 idea more indirectly - like mine. I like this theme because it's very open.
@dopamine: I like it a lot. I know you're from NSL, and I took a look over there. I can see why you'd have to change your idea of what a banner is now that you're doing stuff over here. The NSL banners are nutty good, but really different from what people expect here. I still think it's possible to incorporate that NSL style into Magic banners, though. My only suggestion (based on what I've seen from you so far) is BIGGER! Go ahead and make your banner the 600x100 and use higher-res art.