There is a built-in screenshot tool called Kano Screenshot. It doesn't have a graphical interface which means it only runs from the command line but it's easy enough to work out, simply open up a terminal and type:
kano-screenshot -d 15
the -d stands for delay and the number that follows is the number of seconds until the screenshot is taken, in this example, 15.
If you wanted to, you could run kano-screenshot without the delay option but that would only end up taking a screenshot of the terminal.
The images are stored in the home folder by default.
For an additional list of options with the app, simply type