Well, it's disappointing that there is no solution. However, I'd still like to share what I did to improvise (might actually help someone who's stuck on the same issue):
1) The Sprite can be scaled to fullscreen (or layout Scale size to zoom it in to fullscreen).
2) Take a screenshot of the Window (follow the "examples" of Construct 2 on how to take screenshots)
3) Download the screenshot at the same time (also there in the screenshot examples)
4) Return back to actual image size (or Scale back to normal)
--the user will not really notice the scaling since it will be done automatically by C2 in miliseconds--
5) The users can find Downloaded image from the notification panel, and then easily 'share' the downloaded pic (manually) on Facebook, etc.
Hope this helps