Properties:
* Uses a colour map: available colors are from 2 to 256 in powers of 2.
* So each pixel uses anything from 1 bit to 8 bits.
* One colour may be designated as transparent.
* Uses Lempel-Ziff-Welch compression which identifies patterns in the byte stream to compress the result.
* Used to be patent-encumbered, but the patent has expired.
This means:
* Low number of colours available
* Otherwise exact reproduction of image
* So good for text, bad for photos
* Minimal transparency control
