Sometimes I need to get some screenshoots while I'm running live CD's without screenshoots applications, I solved this problem using the xwd command:
In a terminal window or in a virtual console you can run the following command:
# xwd -root > screenshoot.xwd
this command will make a dump of the current image on the XServer.
If you want to see the result you can convert this file with the convert utility
# convert screenshoot.xwd screenshoot.png
Now you have a standard image file that you can open with a lot of graphic programs or with all Web Browsers.
Gg1