With the compact keyboard setting, you use the right bracket ] to hide the screen elements, and the left bracket [ to take a screenshot. If you've got your speakers turned on, you'll hear a little *click*. If you want to hide the names (which Squenix requires if you don't have permission from the other people in the screenshot...la la la la...) type in /names before you press the right bracket and hide the screen elements.
My recommendation is to get an application like FRAPS, at www.fraps.com. It allows you to take screenshots at the resolution you're playing at, and they come out very nice. FRAPS 1.9D is freeware and has no limitations or banners.