The greatest drawback to XBM images is that they’re monochromatic-in other words, black-and-white, though Netscape renders them as black-and-gray. But this is also something of an advantage to us because they can be represented internally as a string of bits, one per pixel, on or off.
This also makes manipulating them fairly straightforward and not too costly in terms of processor cycles-an important consideration when working with an interpreted language, such as JavaScript. Also important to us, the XBM file’s native format is ASCII text, which can be represented using JavaScript strings.
Ananova 