Hello,
I have the same problem as described before. It occurs after editing an event for the first time. Text and the image can be entered quite normal as for Pages. But after saving, the editor text area contains HTML code <img> when there was an image included.
As this behaviour cannot be inspected on the front-end, I have made a screenshot of the Edit Event here:
Screenshot
Using WP 4.4.2 with Theme Twenty Twelve
