Main menu has 2 top-level items.
Mega menu created for each top level item.
Add an image to mega menu #1.
(Mega menu #2 is obviously not visible as we are editing Mega menu #1)
Move drag image with mouse click within mega menu #1.
Image disappears and red alignment lines are shown.
Image appears to be gone. Can't see it in page outline structure to verify!
Open mega menu #2 and image is visible.

Mega menu surfaces should be completely independent of each other.
The only way I have found to move the image is by moving it with keyboard keys 1px at a time.