I sometimes post events that run for several days, like the run of a play. (Naturally they are entered as Event Announcements and not Member Activities.) I write the date range in the title (e.g. March 1-5) but of course an event on Glocals can only be one day long. So I first post the event date as March 1, and then every day, I change the event time to the next day.
I learned early on that if I managed to remember to change the event time before its start time (e.g. 6pm on 1 March for an 8pm event), then the event is updated, no problem. But if I change the event after its start time, then I lose the event image (everything else is preserved).
Definitely a bug, although not a major one. I can see why it hasn't really come up before. Curious to see if anyone else has had this, however. Important to note that event image is not lost in duplicated events, only in events where the date is changed after the entered time.
Of course the best solution would be the option for multi-day events, but given the nature of Glocals events I can see, too, why that is unlikely to be useful most of the time.