You can take a snapshot of your screen or of an individual application window by using the Take Screenshots utility. Start it by pressing Print to take a screenshot of the entire desktop or by pressing Alt–Print to take a screenshot of the currently active window or dialog.
The screenshots are automatically saved to your ~/Pictures directory.
You can also use GIMP to take screenshots , In GIMP, click File › Create › Screenshot, select an area, choose a delay and then click Snap.