I know in Photoshop that when saving as a BMP, it has checkboxes (below your new filename?), one of which is to save alpha channels -- this is what makes the BMP 32-bit instead of the expected size of your regular 24-bit bitmaps.
EDIT: I don't know about gimp, but you might try disabling transparency or alpha channels before saving. If you have an option for optimizing your image for the web or similar, there might be a setting under saving as gif, if that is the selected choice. I'm just reaching here. Also, you might be able re-save your BMPs with a simple image editor like the default windows paint (doesn't support alpha channels) or whatever you got.
REEDIT: Oh, check out ArmyDude's tut here,
http://www.droidforums.net/forum/droid-x-hacks/92469-step-step-diy-boot-m-logo-pics.html, just before hex-editing.
PS Amazing work MG and the Dr for the previews thread. I have no idea how much work went into that, but I don't doubt your left-click finger is a bit sore!