Is there a way to take a screenshot of what is on screen by being remotely logged on that machine with ssh?
Vnc is no good, since AFAIK it cannot take ownership of an existing X session.
You might want to try this:
It will save the screenshot to screenshot.xwd which GIMP should be able to open. Check out the xwd man page here: http://linux.die.net/man/1/xwd