If you want free graphics programs I don't think you can go wrong with Inkscape and Gimp. Both are free and are, in their own way equal to Illustrator and Photoshop respectively. For Gimp we have plenty of tutorials, and after you go through some of those, trying some of the Photoshop ones (will require some translation and fiddling around to create effects that Gimp doesn't do) can yield great results. You can read more about these programs in the sticky thread at the top of this forum. Browse through the Tutorial forum to see what is available for Gimp and Inkscape and you will see what I mean.
Both of these progs will allow you to take the image of your poster board map and allow you to work with that. I must say that it will require some work learning the ins and outs of these programs, but not necessarily as much as you think if you follow some of the tuts, and you will be surprised to find out how nice a map you can make.