Currently it seems that dragging and dropping an image from a first note to a second note is impossible (at least on Mac, I haven’t tried in iOS).
As I understand it, it seems that as you initiate the drag the text cursor becomes active in the first note, and remains so while you attempt to drop the image in the second note. My various experiments with Agenda suggest that to successfully drop an image in a note (from eg the Finder) the text cursor should be active in that note. I might be completely wrong on this though.
It would be very useful in my opinion to enable drag-n-dropping images between notes.