I have a tip regarding that!
All you do is set a time delay, usually 2 or 3 seconds is adequate. So you click "take a screen shot", you then have a couple of seconds to click on something and only then is the screenshot actually taken. If you select the option to include the mouse pointer, then this also allows you to point at something with the mouse pointer. I am using Linux, some Ubuntu and mostly KDE Kubuntu. But as far as I know most screen capture application can do these things.