I think I would go with Inkscape (also free) instead of either Autorealm or the Gimp. Inkscape is a vector graphics program, but with an easier interface than Autorealm (at least as of the last time I looked at Autorealm about 10-15 years ago) I think that the learning curve is a bit steep with the Gimp if this is going to be a one-offish sort of thing. Don't get me wrong, the Gimp is a great prog, but might be a bit complicated for what you need.

Good luck with your project and Welcome to the Guild, The Mappiest Place on the Web.