If you use make an image backup of an 8gb SD card, you get an 8gb file. Which is biggish, especially if you have several of them, on for each Pi.
So I used zip. I don't think the version of zip that I have, likes files over 2gb. Ditto rar. Gunzip worked, but it only reduced the size from 8gb to 5gb. So I tried bzip2, and that worked great - the 8gb files became 0.5 gb, which is more what I'd expect, since most of the SD card is empty, so should compress down a *lot*.