Photoshop is the best, but it's very expensive. For a cheaper solution, try imikimi.com atau picnik.com. They don't look as good, but anda can still find some cool effects. And if anda use them both for the same project, it looks almost as good as Photoshop.
I Don't know WHERE but I can tell anda HOW, anda need rather GIMP atau Photoshop to make it look all cool, I recommend getting GIMP Because anda don't need to STINKIN' PAY FOR IT!!!! :) then go to YouTube for a bunch of tutorials, anda can find many, MANY on there! Trust me! then anda can make your wallpapers~♥