What is a .bmp file?

BMP is an uncompressed image format that stores every pixel as raw colour data — no compression, no quality loss, and no reason to use it in the modern era. The format time forgot. A 12-megapixel photo saved as BMP weighs roughly 36 MB; the same image as JPG is under 5 MB. Microsoft introduced BMP with Windows 1.0 in 1985, and it has barely evolved since. You will encounter it in legacy Windows applications, old scanner software, and screenshots from people who do not know about PNG. There is almost never a reason to create new BMP files. Convert to PNG for lossless quality or JPG for smaller files.