One of the really cool features of Windows 8 is the Screenshot feature. Press Win + PrintScreen and you will get a screenshot automatically saved at %userprofile%\Pictures\Screenshots. It will be named as 'Screenshot (#).png' where # indicates the screenshot index.
This screenshot index value is stored in the registry permanently. Even if you remove all your screenshot images, the next screenshot you take will have a higher index. Here's how to reset the screenshot counter.