BackTrack 5 Linux is world most popular penetration testing distribution it is compiled purely with Penetration testing tools. TakeScreenshot is popular image capturing application available under all Linux distribution but isn’t available under BackTrack 5.
TakeScreenShot is still available in repository in gnome-utils package, you can follow below guideline.
How to install TakeScreenShot on BackTrack 5 ?
aptitude install gnome-utils
To access application;
Applications -> Accessories -> Take Screen Shot