BMP might not be allowed. That's a prehistorical format and resulting file size is much larger than it needs to be. I do think PNG images are allowed which uses non-lossy compression. However JPG is the best choice in most cases and gives the best quality to size ratio. Please do upload the largest and best quality images available though!
By the way if you have a lot of images and don't want to convert them one by one you can use a free tool like imagemagick/mogrify.