This is not possible since the website is forbidden to access anything on the user's computer, unless you've added a plugin or extension specifically for this job. It is however possible (for web browsers which are up to date) to take a printscreen and paste it immediately to the text editor (Ctrl + v). You can also make a snippit with Windows' Snipping Tool (Windows 7 and up) or any other similar tool found online and immediately paste that to the editor.
Not as pretty as a button, but this for example should do the trick:
- Windows button + d to go to the desktop
- Printscreen
- Windows button + d to open all windows again
- Select the editor
- Press Ctrl + p to paste the screenshot
