I'd like to suggest adding more entries to the default dont compress list (DEFAULT_DONT_COMPRESS in loadparm.c), namely:
.xz .txz (http://tukaani.org/xz/, new fmt replacing lzma and 7zip)
.lz .tlz (http://lzip.nongnu.org, another fmt along those lines)
xz is being used in rpm payloads these days, among other places.
It compresses better than bz2 and typically decompresses at about the speed of gz. (For that matter, I also suggest distributing rsync as a .tar.xz. :)
On another front, I don't think .png files be usefully compressed either.
Thanks for the suggestions. I'm adding them.