Want to knock together a book cover and don't have the dosh for Photoshop? No worries! GIMP is here for you. GIMP is shareware and shareware means free and legal!
I don't pretend to be a GIMP guru, but I've managed to make it jump through some hoops for me over the years. The image below is a WIP map for one of my stories that I created on GIMP.
Download page: http://www.gimp.org/downloads/
Mac downloads are at the top of the page, Windows downloads a little further down.
Since GIMP is an open source shareware platform, there are lots of different 'builds' available. If anyone has a favorite build for Mac or Windows, please feel free to comment that information.