NifSkope is an option, but it will not look the same as in game. Steam is not the only option to take screenshots, your best bet is to install Fraps, MSI Afterburner or whatever program that's capable of taking screenshots. Set whatever hotkey you like and problem solved. Screenshots will go directly to a folder you specify. Method 3 of 4 here http://www.wikihow.com/Use-Fraps-to-Take-a-Screen-Capture shows how to set up screenshots, you can disable FPS counter and recording if you don't need that.