You should go ahead and get a program that's more of a use for graphics involving banners, avatars, buttons, etc. Something like Adobe Photoshop CS2. I've never downloaded GIMP, so I don't actually know how to work with it, nor give you advice on how to use it for the purpose of creating better graphics. If GIMP is as good as CS2, then you should just keep practising and eventually get better. If you truly want to create graphics in a short amount of time, search for a tutorial. I suggest that you check out NF (Naruto Forums), then go the Graphics forum, and you'll get outstanding tutorials on how to create many types of graphics with different styles. They're very helpful, I even have tried some out and they've worked well enough for me.
By the way, how long have you had the program you're currently using?