I recommend GIMP.
It's easy to use and powerful, plus it's completely free.
Then there's Photoshop, which is a lot more powerful than GIMP, but it costs hella lot.
And it's more like to advanced art program users, because it's a bit more complicated than GIMP.
Ok ill use gimp then, thanks. Oh and also i used gimp at a friends house once, does it matter how big the picture is, will it shape it up if you try to use it or is there a format for the head texture size?