I was having a tough time running some functional tests against Firefox using Xvfb on our Jenkins machine here at work. I couldn't figure out why the tests were dying. I looked up how to do a screen capture and went to some page that suggested an xvd command well we didn't have that installed on our box but we did have ImageMagick. Turns out you can run an import command to take captures for you and save them to a file. Then you can scp them off the machine.
The simple command is:
import -window root screenshot.png
Here's more info if you need it.