I use a program called "Lightscreen" and it works great. It's simple and has some great options. It's open source and a 5 MB download for Windows.
Once installed just hit the "Print Screen" button on your computer ( buttons can be custom set also ) and screenshots will be saved to the folder of your choice without any interruption to your game play.
Where <myimagename> is an words/numbers, without a file extension (like jpg, as it will be added by the system)
I heard somewhere that Command line codes like this one you posted are considered to be hax and there for you should remove it so it doesn't reference any hax in this game before a mod comes in here and closes this thread