The Now Playing page chooses the first artist that has a picture. @mike has said we will get the same behavior as on the Display, where it cycles through artists.
In the meantime I noticed a subtle bug:
I was listening to Avec Les Temps by Giovanni Guidi. I have added all five credited artists as Primary Artists, all have pictures, but NP chooses Guidi who is listed first. Then I played another album, Blues Connotation, that includes Roberto Cecchetto who is the third artist listed on Avec Les Temps (the other artist on Blues Connotation, Giovanni Maier, does not perform on Avec Les Temps), and NP displayed Cecchetto. But then, when I went back to play Avec Les Temps, NP showed Cecchetto instead of Guidi. Like it had a memory. I shut down the iPad app, restarted it, and now NP showed Guidi as it normally does.
One of those bugs where if program management would specify they want this behavior, the devs would say, no that’s not possible.