It's pretty much impossible to completely prevent copyrighted works like described in the OP from getting uploaded unless the uploader has seen it before. Most are familiar with Nintendo and other popular characters but sprites from games like Maplestory are a lot harder to catch.

I personally wouldn't have known the flower was ripped from Maplestory and I've actually played the game.

So if you notice copyright-infringing graphics report the person, take a screen-shot and email it to the Classic's (or Era's if that's where you notice it on) support email and it will be handled appropriately.