Here is how to take a screenshot on an iMac or a portion of it (or any other Mac running OSX)

applemacscreenshot

 

The screen shot feature is one of many features built directly in to OSX.

Capturing the Whole Screen

Whilst holding down [command] and  [shift] press [3]

Capturing a portion of the screen

Whilst holding down [command] and  [shift] press [4]

You will see the mouse pointer turns in to a cross hair.

Simply Left click and drag the area you wish to create a screen shot from and release.

Using both of these methods you will find a new file on your desktop with the newly create Screen Shot you have created.

Screen-Shot-2013-07-27-at-2.44.59-PM