Got this as an email the other day on my University of Minnesota account...it talks about the compression used. Haven't read up on it, though.
"The Mars Exploration Rover (MER) is currently landing a pair of rovers on Mars (one landed last week and the other will be landing soon).
Well over half of the bits transmitted from the rovers will consist of compressed image data gathered from the unprecedented nine cameras
on-board each rover. This compression is based on the ICER and the LOCO [1] image compression technologies. LOCO was developed by Dr. Marcelo Weinberger and Dr. Gadiel Seroussi from Hewlett-Packard Laboratories and Prof. Sapiro from the Electrical and Computer Engineering Department (while he was at the HP Labs before joining the University).
The JPL/NASA hardware implementation of LOCO on-board the rovers is used when maximum geometric and radiometric fidelity is required. The LOCO technology, patented by Sapiro, Seroussi, and Weinberger at Hewlett-Packard Laboratories, is also the core of the international standard JPEG-LS for
the lossless and near-lossless compression of still images."
[1] M. J. Weinberger, G, Seroussi, and G. Sapiro, ``The LOCO-I lossless image compression algorithm: Principles and standardization into JPEG-LS,'' IEEE Trans. Image Processing 9, pp. 1309-1324, 2000.)