The ix ap format was designed specifically for small icons. It is in simple ASCII but supports multiple colors, even a transparent color. It is supported on many platforms and most browsers. It is not space efficient, but because it is always used for small images, this is usually not a problem.
An X-formatted color bitmap image. (.xpm)
XPM (X PixMap) is an ASCII image format used by the X Window System. It was created in 1989 by Daniel Dardailler and Colas Nahaboo working at the INRIA, France, and was later enhanced by Arnaud Le Hors. It is intended primarily for creating icon pixmaps, and supports transparent color.