You might also mention or suggest what kind of style you're looking for in the final product - best way is probably with a link to a sample map whose style you like, but you could describe it a little. Might also be nice to know if you're looking for a particular file format, vector vs. raster, or any other little details.

There's also the fact that your existing map is already pretty good - which probably means the less skilled artists (myself definitely included) will be more reluctant to take a stab at it.