Hi Alvin,
Nice post. I can reproduce this on my machine with InfoPath 2013. I'm guessing it's because the screenshot image exists as a sub-xml attribute value. In other words, the value of the rich text didn't change, just some child attributes were added. Can't see a way around this since it seems like default functionality.
However, I do recommend replacing it with a picture control. You can still use copy-paste to add images to a picture control, provided it is selected first. I just single-right click and hit escape since clicking causes it to launch the browse dialog.
Thanks for posting,