That's what I tried too, and encountered the same problem. I think the part of the process that's missing is either conversion of all colors to the SPMBT palette (not sure how to do that using GIMP), or that the 36-image converter app doesn't properly convert the file.
Don's full explanation of how to do this using Paint Shop Pro can be found here:
http://www.shrapnelcommunity.com/thr...rt=&PHPSESSID=