Almost all the possible Mapping applications can save as a JPG, in come cases as PNG and/or BMP. Some of those apps can save in any format.

While I'm not a GIMP user, this is a robust, script-ready raster application much like Photoshop and as a GNU product, its totally free. While it is complex, we have tons of tutorials to get you started right away, and actually creating high end maps right from the start. Look for Random Coastline tutorial, by RobA, its one of our best tutorials on GIMP on the site and worth taking a look. In step by step fashion, you'll learn discreet and primary tools to achieve professional results. But that's for photo realistic and painterly style maps, if you need something more "clean" perhaps a strictly vector tool like Inkscape (another GNU product = FREE) to make that kind of map.

Many of us mappers use more than one application for different parts of the map. Labels are sometimes easier using Inkscape on maps generated in GIMP, for example.

Some guys are making useful maps out Microsoft Word, it depends on what you have, what you want, etc, etc.

We each have our own favorites, and each use different workflows, so its a very individual thing - which software we find easiest to map in...

GP