Aqui va la definición flojita:
<file format, graphics> Microsoft Windows bitmap format.
Bmp files may use run-length encoding.
This is the only graphics format where compression actually
enlarges the file. The format is widely used nonetheless.
el mejor diccionario en línea que conozco)
.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.
Nevertheless, the simplicity of BMP and its widespread familiarity in Windows and elsewhere, as well as the fact that this format is well documented, makes it a very common format that image processing programs from many operating systems can read and write.
"This is the only graphics format where compression actually
enlarges the file. The format is widely used nonetheless."
JUAS JUA JUAS...
De todas formas cualquiera puede "crear" su propio formato gráfico a nivel tan simple como lo es el .bmp.