If you have imagemagick installed (which you should), you can do many magical
things with images. Check out the main website and familiarize yourself with utilities that come with it (e.g. resize, convert etc.).
The command import filename.png
gives you a mouse pointer to click on the desired window to take a picture to filename.png.
To capture the entire screen, do
import -window root filename.png
We can set up an automatic timestamp in the filenames so that they do not get overwritten (provided you take not more than one picture every second):
import -verbose -window root capture-$(date +%d-%h-%Y-%H-%M-%S).png
In this case,
sleep 5; import -window root file.png;
will be more effective.
Taking a screenshot of screen 0 (of DISPLAY 0)through some shell: If you log in through ssh and want to capture a screenshot at that moment, issue:
import -window root -display :0.0 -screen capture-$(date +%d-%h-%Y-%H-%M-%S).png