When you write PC tutorials, being these for Linux, Mac or Windows, well the same applies now for Android or iPhones, you need screenshots
If you are working with Linux imagemagick is a great option, but that is the geeky way ;). I can see that Linux is now becoming more and more user friendly. And maybe Ubuntu is still the king in this area. So, instead of a command on the terminal using
imagemagick we are going to use Shutter. If you are using ubuntu install it from Synaptic or the geeky way.
sudo apt-get install shutter
Once installed, it will be available from the menu, under Accessories. Open it, and then just select, selection draw a rectangle with your mouse and press Enter when you are done. Or ESC to abort. That is if you selected the advanced selection tool. Using the simple one, just select an area, and it will be captured.
I try to avoid graphic tools, I still prefer the command line, but shutter is one I will definitely use.