Most games come with a screenshot command, check to see if there is one.
All depends on the OS you are running the game on. If it is windows then the print screen should work. Just try pressing it a couple times really fast. Then paste it to a paint program. If not, then there are programs out there that can do this function for you by pressing speical keys. But I don't know the names of these software titles, at this time.
prob something like.. /screenshot nameyourpicturehere (do not add .jpg or anything) or /ss nameyourpicturehere (do not add .jpg or anything) Read the manual (heheh)