Thanks for the replies, I've tried both techniques but no luck.
I've attached the 1024x1024, 24bbp, BMP file. I had to change the file extension since uploading .bmp isn't allowed, just change it back.
I surely hope you can figure it out...
By the way... Does BMP2GMP work with grayscales or can you give a map a colour (e.g. a satellite image) overlay?
It doesn't really matter up to this point because I've tried both grayscale and satellite images and neither of them work.