How to screenshot or pipe the output from xscreensaver?

Some outputs are really great, IMHO. I have read all xscreensaver manual, but I cannot get it to work so I run a xscreensaver animation and then get a PNG (or TIF, or whatever). How can I make a virtual display so it can be screenshot? Or maybe can I somehow pipe out the output to a PNG/TIF file? Or is it possible to run each animation as standalone command?