If you have a Macintosh, this feature is built into the system:
1 – Press Shit-Alt-3 to take a snapshot of the screen (it will save to your desktop automatically)
2 – Open the file by double clicking on it (it will open in Preview)
3 – Go to the ‘File’ menu, select ‘Grab’ and ‘Selection’ – a tool will open allowing you to draw a box around the part of the screen you want to save
4 – Draw the box around the bit you want to save and then choose the file type when asked.
4 – That’s it – the screenshot you selected is saved on the desktop