Here's another way:
Alt+F2, then type in the box:
xwd | convert - ~/lq.png
This will take a snapshot of the entire active window, and you will have a lq.png in your /home/yourname directory. The lq.png is just a name using the LQ forum as an example. Name it anything you wish .png.
Now, if there is more to this than you want in the screenshot then go again to Alt+F2, and type in the box:
This will give you a crosshair pointer so that you can cropjust what you want on the page. Just go to a section of the active window, click and drag until the dotted lines cover all you want. after you let go of the mouse, you will have a llq.jpg file in your /home/yourname directory.
The file extension is not that important - its your choice. jpj or png.
Now lets say you want to change the size to upload to a forum, or even for use as an avatar. For this we have the convert command:
convert -geometry 800x600 /home/yourname/lq.png
Change that 800x600 to whatever you want.
All of this from the command line.