You can map printscreen (or any key combo) to this utility called xfce4-screenshooter - you can use it even if you don't use xfce desktop. By default it will ask you what to capture (entire screen, active window, select region etc) and by default it will ask you what to do with the screenshot (save to disk, copy to clipboard etc) but you can add switches to the command that gets executed when you press printscreen. For example, "xfce4-screenshooter -c -f" will take a full screen shot and copy it to the clipboard.
In the mint repositories the package is called xfce4-screenshooter, i'm suspecting it will be the same in all ubuntu variants.