Yeah, there's nothing surprising. GD2 doesn't support BMP files correctly. Some of them will go through, which is why it's officially accepted by SMG, but some of them won't. Integrating a third-party plugin to enable pixel-perfect import of BMP files can be done, it's in my to-do-list, but nothing urgent I'm afraid. Tell your users to convert BMP files to a better format. The best lossless format is PNG. It will divide their filesize by 2 to 5, guaranteed, with absolutely no visual loss.