Yeah, I've tried everything available, but it keeps complaining about the bitmap format not being understood.

Tried MSPaint/PBRush, GIMP and Paint.NET. Running Windows XP. None of the bitmaps can be changed, saved, then imported. The tool complains.

You might want to add this problem to the to-do list, to fix the bitmap incompatibility. I'll wait for a future release and hope I see some support. Great tool regardless, though.