Quite a few image editors will open a .BUM file, which is based on .BMP format but without any blue channel information. Paint Shop Pro does for sure... You can then save these to any format you like. BUT the red and green channel information is offset compared to the original greyscale. Whether or not this matters depends on the bump map in question; some will work well enough, others will not. General "rough texture" type maps will usually be fine, but one which is trying to generate well-defined features will give a fuzzy result.