1. A method for compressing an indexed color image on a computer, the method comprising: identifying a pixel color for a pixel; identifying a left neighbor color for a left neighbor of the pixel; identifying an upper neighbor color for an upper neighbor of the pixel; comparing the pixel color with the left neighbor color and the upper neighbor color; selecting a probability model from a probability set, the probability set including at least two probability models, each probability model including at least two probabilities for the pixel color; encoding the pixel color based on the comparison using the selected probability model; and updating each probability in the selected probability model to reflect the pixel color.