I just tested Gimp 2.10.12 with the latest version of AH and can confirm that HT has fixed the issue the game was having with its bmps. Both the 24-bit and 32-bit bmps output from 2.10 now view correctly, without the "shifted a few pixels to the right" problem.
Paint.Net's 24-bit bmps also view correctly but its 32-bit bmps are still offset. Hopefully that will get sorted next time, even though no one is currently using this program for skinning. I tested it because its the only other free editor I could find that can output a 32-bit bmp. The offset seems to be caused by the various formats of bmp header produced by different editor programs. So although other editors out there may also have similar problems, for future AH skinning the above two editors should be the only ones we need to worry about.