the key is f12, there will be a .png file (i think its .png) created in your riot folder. If not in the main folder its in one of the other sub-folders. Be sure to rename the files otherwise they will be overwritten in your next game when you take a screen shot.
@Digitalneo:
When you take the screen shot, your computer eats up some resources gathering the image and saving it to your harddrive. Unless you have a beast of a machine, you should notice the drop in Frames Per Second. It will feel like a small lag spike.