Ok, the MM.jpg was an old MapMonkey background image. It was also used for other GPS screens in the BMV2 skin. For some reason the BMV2 skin did not include that image, and the older RR would not report it. So, until the new checks were incorporated into RR, it would just load the skin fine with no errors.
To fix it: Open the external_GPS.skin and replace the 3rd line with this:
That will take care of it, and the screen will load without any problems.