There are many reasons to take screenshots in M&B, for mod development, personal amusement..etc. This Tutorial will explain how to take screenshots in mount&blade.
First, you may take screenshots in-game using the Ctrl + Insert key. After the screenshot is taken it can be found in the directory:
C:/Users/Yourname/Documents/Mount&bladescreenshots/
However I prefer using
FRAPS. Fraps is a application that allows the user to take screenshots and small videos (if you are using the trial version). The full version allows more advanced settings for the video and screenshot capture.