A simple way of taking a screenshot of a given window under Xorg is to use xwd with imagemagick. To take the screenshot, open a terminal and type:
xwd > my-shot.xwd
Then, select the window you want with the crossed cursor. Finally, convert my-shot.xwd into a more common image format with:
convert my-shot.xwd my-shot.png
If the screenshot needs to be cropped, mtpaint can be useful.