Paint shop Pro and Photoshop are the ones most people use.
PaintShop is the cheapest ad you can find a demo version on theyr website:
http://www.jasc.com/products/paintshoppro/
But you need to use the Palace palette if you want to do a full edit in this program. The easy way to do this is: Edit your picture in paintshop pro, or your favorite picture editor. Make sure everything you want to make transparent is one extreme color like 100% green. Save this picture and use Galen to import it to the Palace client.
Or look up one of the tutorials for Paintshop pro in the links/Avatars Props & Dolls section :)