By default printscreen or alt print screen will get the whole desktop or the application in focus, respectively. But what if you want to select a portion of the screen?
This is a nicely integrated solution
sudo apt-get install compizconfig-settings-manager
Then open the settings wizard and go to Extras and check off the "Screenshot" Option
You should be able to hold the Windows Key and drag to take a screenshot now (does not work on Linux Mint 18 though).
You can also change the initiate key by clicking on Screenshot in Compiz:
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