A BMP or bitmap image is Window's native graphics file format.
Bitmap images are uncompressed, so they can be very large.
That takes up a lot of room on your hard drive, and you'd never want to put it on a webpage. It would take forever to download. A JPEG's file size can be a tenth or less of the file size of the original image. When you create a JPEG of an image, it's up to you to decide which is more important, image quality or file size. Of course, the higher the image quality, the larger the file size.