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