When I'm at work I use GIMP which can be used in either linux or windows. Do a search for GIMP on google to find out more.
Oh, forgot to mention, I hate gimp, and use Photoshp at home.
The biggest complaint I have with GIMP so far is when I use the hotkey for the eyedropper and select my color, I then have to manually click on the toolbar to select my pencil due to a popup. With photoshop on the other hand I always have my 2 fingers on the hotkeys for eyedropper and pencil. Makes for quick editing.
Never tried Graphics Gale,
I use cloverpaint on my Zaurus SL-C700