.BMP or .DIB (device-independent bitmap) is a bitmapped graphics format used internally by the Microsoft Windows graphics subsystem (GDI), and used commonly as a simple graphics file format on that platform.
BMP files are usually not compressed, so they are typically much larger than compressed image file formats for the same image. The typical true-color bitmap size in bytes can be calculated as: (width in pixels)*(height in pixels)*3. So an 800x600 image will occupy almost 1.5 megabytes. As such they are generally unsuitable for transferring images on the internet or other slow or capacity limited media.