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:
xwd -out screenshot.xwd -root -display :0.0
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
Assuming you have ImageMagick installed on the remote and local machine, the X server on the remote machine is running as the user you are connecting with, you can take a screenshot remotely and display it on your local X server like this:
ssh email@example.com "DISPLAY=:0 import -window root -" | display
This is very convenient as you don't need to copy any temporary files.