Before using screenchot custom action you need to install sandbox solution and activate the same name site feature
ScreenShot is a Custom Action which is added to pages and forms with RTE ribbon component. To use it's functionality you need:
1. Open the page, choose "Page" tab and click "Edit" button (or edit list item with RTE field).
2. Make a screenshot or copy any picture to the clipboard (e.g. press Alt+PrintScrn or select picture area in any image editor and press Ctrl+C).
3. Chooce the place in the RTE area where you want to paste it and press Ctrl+V.
4. Enjoy you screenshot =)