Don't know if this is what you are asking but when I want to capture a screen from my computer I hit Print Screen (PRT SCRN). It should be on the top of your keyboard, to the right of the number keys. What that does is put your screen onto your clipboard (a form of memory on your computer) From here, you can put it into an email or document, open it in Photoshop etc. Once you hit > then open the software and hit > That will paste the capture into the program.

So, if playing Poker Stars or Party Poker... and you get a killer hand that you want to keep a capture of, before the next deal, click print screen. Open Photoshop and open a NEW document. That should already be formatted for the size of your screen and the image that was captured. Click Control + V and it will paste into that document. Then save as a new JPEG image.