Using the print screen key works, but it will only allow you to save one picture at a time to the clip board. A lot of people use a print screen utility, such as
Fraps or
Gadwin to take pictures in their game. This will allow to take as many pictures as you want to without having to minimize the game and paste each picture as you take it. All the photos you take will be saved to a file created by the program, which you can then open in an editing program and resize to the forum specs.