What is the size of the different graphics you need ?
Do you need different size for platform or is it using some kind of tiles ?
BTW, you should take a look at http://opengameart.org/
From the FAQ :
What's the purpose of this site?
If you've ever browsed Free/Open Source game sites (such as The Linux Game Tome, you'll notice that a fairly significant number of the games available suffer from what's lovingly referred to as 'programmer art'. There are, of course, some notable exceptions to this, but it's clear that, for an open source game to produce good art, it has to become large and popular enough to attract artists.
Unfortunately, many fun and well-designed games never reach this point and are thus stuck with placeholder art, which ultimately detracts from their appeal and popularity. Furthermore, it's not unheard of for open source projects to rip their placeholder art from commercial games, which is illegal and could conceivably result in a lawsuit.
The purpose of this site is to provide a solid (and hopefully ever-expanding) variety of high quality, freely licensed art, so that free/open source game developers can use it in their games.