I would suggest that you use JPG/PNG/GIF instead of BMP files for images. BMP files are usually very large and this would not have a good impact on your website (disk space will be used up very quickly and page load time will be slow as it downloads those images.)
However that being said, I have seen it when images are uploaded and when Magento is trying to resize them the correct GD libraries are missing for PHP. I would have a look in your php /apache error logs and exceptions.log to see if this is the case.
If its reporting something like image2wbmp is undefined, or getimagesize perhaps then GD is likely not properly configured on the PHP setup.