How to take Ordinary Screenshots
There are several options for taking screenshots on a Mac. They save by default to your desktop but you can probably change this if you want.
a.) Full Screen
To just take a screenshot of the whole screen (you can crop it later using GIMP or Photoshop or even Preview) just use:
b.) Selected Area
To take a screenshot of a selected part of the screen use:
After you have pressed these 3 keys you will be able to select the area you want. As soon as you let go it will take the shot so there isn’t much scope for resizing.