This tool reads your BMP image directly in the browser and encodes a JPG copy, so there is no upload, no queue and no waiting for a server. It works on desktop and mobile.
BMP files are uncompressed and can be many megabytes for a single picture. Converting to JPG typically shrinks them by 90% or more so they can be emailed or uploaded.
BMPs from old scanners and screenshots compress extremely well; a quality of 80% is usually plenty.