@KamikaseWorm - GIMP ([url]gimp.org[/url]) is free image-editing package you could use. Lots of Photoshop-esque tools, you can export as *.PNG and you can convert your images to indexed colour (which is needed to get maps to work in the game).
Check out this page, it should help you out: [url]worms2d.info/Colour_map[/url]