The GIMP is a big pain in the ass, but its pretty good for freeware.
Just remember that if you use Targas, you must remove ANY alpha channels that exist in the image, or you will crash Wc3 when you load the image.
Also, Vista comes with a Targa reader as well. I haven't tested for any programs that could save them as Targa, except Gimp and Photoshop.