for doctoring GT4 photos, i use picasa. its hosted by google and its free. its very easy and probably the best suited for newbs. if thats what you are . then there is photoshop, which is more complex and has better tools. and then there is GIMP. which is like a photoshop lite i believe. good luck!
Well, "photoshop" is a program itself. But yea, if you are kinda new to editing, gimp or picasa, but photoshop and paintshop pro are better but more difficult to use.