One way is to hit the Print Screen Key (PrtScr) which is on the top row to the right. Then Alt-Tab to minimise the game, open a graphics app like MS Paint, then paste (Ctr+V) the image into it and Save it.
Another way is to use Frapps (Google it). It will do it automatically without having to go through all of the steps above.
Make sure you download the one at the bottom of the page. If you download the Pro one, it will give you a watermark.
One thing about taking Printscreens normally is that sometimes your game will minimize or you have to minimize your game in order to paste your picture somewhere. Without Gadwin you would have to paste your picture in a photo editing program before you could take a new picture. With Gadwin you have the option to store your pictures in a file so you can edit them as soon as you're done playing.
Under 'Destination' you will find a bunch of options.
You can copy your pictures to the clipboard by selecting 'Copy Captured Area to Clipboard'. If you do this one only you will have to paste your picture in the editing program before you can take a new one.
If you select 'Copy Captured Area to File' your pictures will be stored in a folder (in My Documents) and you can continue taking pictures and edit when you're all finished playing the game.
I currently have both options (first and last) checked.
If you don't want the PrintScreen button to be the "hot key" (the button that takes the picture) you have a list of options to choose from if you click on the 'Preferences' tab.