GIMP is the closest you'll get to Photoshop and it's 100% legally free:
http://www.gimp.org/
You can also use Photoshop brushes with it.
If you need any help with Apophysis, just leave me a note, all of my wallpapers were made with it.
Also, you can draw AND edit photos with Photoshop or GIMP. You can even paint a picture just like you were using real paint brushes.
YouTube is a great place for videos on how to use GIMP as well.