A bitmap is a grid of coloured dots (pixels) which combine to make an image. BMP files record such images for display on a computer screen. Unlike other bitmap storage formats (see JPEG), BMP files are fairly primitive, and make only a modest attempt at compression. This means that large bitmaps tend to result in large BMP files. Nevertheless, BMP files are common on Windows systems because BMP is one of the standard Windows image formats. BMPs are not programs and cannot be infected with a virus.
from Sophos' V-Files
But see also Steganography.