Angstrom-openpandora-xfce-image-eglibc-ipk-v2012.05-omap3-pandora.rootfs.tar.bz2 is the one you want. The other images aren't there at the moment. It's slightly easier to just have to worry about keeping one image up to date, then having to rebuild 4 of them every time a package gets updated.
Changes between images are really too small and too frequent to bother writing a readme file for, and you can just look at the git commit logs to see what has changed. (and those are also sent to the firmware mailing list)
There's a very experimental sysv image there too, which is only there because someone wanted to test some stuff. (The old firmware used sysv for everything, and this new one systemd, and there are some small config problems that need to be fixed).
The two other images, without the Angstrom part at the start, are simply symlinks to the real Angstrom-* file.
We aren't really at the "testing" stage yet, more at the "trying to get the base system to work" bit. Once that's in place, we can start to worry about actually testing it properly.
Now that i finally have access to the bugtracker, i'll try to get some todo list on there.