I had been adding photos to be associated with various event tags for a person. Then when I opened the Exhibit Log itself and saw all the photos, I thought I could choose one to be Primary. I didn't realize that you have to add the Primary exhibit from the Exhibit Log itself. I had noticed that if I entered the same photo as a link to more than one tag for a person, that photo would appear multiple times in the Exhibit Log, so I assumed that after entering a wedding photo that I had linked to a Marriage tag, I shouldn't enter the same photo again directly from the Exhibit Log because I thought I was doing something wrong if I had the same Exhibit shown multiple times in the Exhibit Log.
I took your advice and entered the photo I want to be Primary directly from the Exhibit Log, and that solved the problem. The same photo is in the Log twice because I had previously linked it to an event tag, but I understand now that's OK.
Thank you for your fast reply! You just saved me from a lot more unsuccessful trial and error.