When the BMP format is chosen, MPC creates 32-bit BMPs with an alpha channel that makes the entire image transparent. While this bug isn't apparent when the images are veiwed in many applications, this is because most applications don't support 32-bit BMPs and simply ignore the alpha channel. This bug can be seen when the shots are viewed with XnView, as discussed here: http://newsgroup.xnview.com/viewtopic.php?t=12806
MPC shouldn't be saving these pics with an alpha channel at all, when was the last time you saw a video file with any transparency?? MPC should save 24-bit BMPs, or at least have an alpha channel that makes the whole image OPAQUE instead of transparent.