You can set a photo on your primary or joint account. However, I’m finding that the iOS app randomly will not display it, showing just the circle with the initial letter inside.
Starling are certainly keeping the photo, since when setting up my joint account, my photo displayed on my wife’s phone but not my own!
On a related note, when I set the photo on the joint account, is it expected to show up on my wife’s phone? It isn’t, but I don’t know if that’s due to the above bug. (It’s no longer showing on my own phone either.)