Instructions
Get to the point in the game where you want to take a screenshot. If you want to take a screenshot of a particular zombie, make sure he's visible on the screen.
Hit the "F5" key on your keyboard to take the screenshot. Hit the key multiple times if you wish; there's no interruption in game play. Each time you press the key, the screenshot is automatically saved.
Navigate to your Left 4 Dead file folder to view the screenshots. Find the folder by opening "My Computer" and navigating to "Program Files." You can find the screenshot in the following folder: Steam\Steamapps\Common\Left4dead\hl2\Screenshots