Screenshots can easily be taken in Killing Floor by pressing F9 by default, which saves them to the
Steam\steamapps\common\killingfloor\ScreenShots folder in .bmp format. However, to take clear screenshots of a map for artistic purposes; without head-up display elements, specimens, or a players hands in shot there are several hoops to jump through.
 Clearing the screen
To hide head-up display elements simply enter the settings menu, and select 'Hide HUD' on the hud tab. To hide a players hands and specimens, load up the map you intend to screenshot in solo. Then during the few seconds before specimens start spawning, swap weapons, pause the game in the middle of the animation while your hands are off screen then open the console. The default key to open the console is apostrophe ('). If you have changed the console key it will likely still be apostrophe on the menu. While on the console run the following commands:
enablecheats playersonly fly
This will stop your hands from coming back on screen, prevent specimens from spawning, and allow you to get better angles of the level. If you want to change the keystroke for taking screenshots the option can be found in the settings menu on the controls tab.