Scrot (SCReenshOT) is a simple command line screen capture
utility that uses imlib2 to grab and save images. Multiple
image formats are supported through imlib2's dynamic saver
modules.

Some features of the scrot:
  - support to multiple image formats (JPG, PNG, GIF, etc.).
  - optimization of the screen shots image quality.
  - capture a specific window or a rectangular area on the
    screen with the help of switch.

scrot also can be used to monitor a desktop PC in admin absent
and register unwanted activities.

WWW: https://github.com/resurrecting-open-source-projects/scrot