I'd agree to the suggestion to keep it in the "vector" world for now. Inkscape is a FOSS alternative to Adobe illustrator.

Keeping it in the vector world means you can create extremely large images that are resolution independent. It also makes tweaking map item much easier while in the development stage. When you are done your playtesting this can then be taken over to a raster application for "prettyfication".

If you want to hand this effort off, please feel fre to post in the map request forum. Several members here (myself included) have worked in this sort of role for board game makers under NDAs in the past, and may help you speed up your development cycle for a reasonable cost.

-Rob A>