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