The screentshots in the assignment handout seem to suggest that they
want you to show the map view in the master and only ever show the
image view in the detail. But, I also did it your way because I
thought it would be more intuitive. I have a segue from the master VC
to the image VC, but I changed the Style to "Replace" and Destination
to "Detail Split", which let me control the detail from the master.
Hope this helps.