Change Game Center profile picture on a iPod Touch 4 (iOS 4)

I got a cool pic on my ipod touch 4 and i can't put it on game center because theres no change picture buttonand i cant see anyone else's picture. I believe its to do with me having ios 4. Im not updating to ios 5 (i got a problem which says my ipod will get all data erased) so whats the easiest way changing it?