@DTS Engineer I think I misunderstood your suggestion regarding the sample project. I switched to using the Asset Catalog to store the images I want and all works well.

Using a MKMarkerAnnotationView is there any way to change the balloon color? Separately, I've looked at the Annotating a Map With Custom Data code and while it requires iOS 18, it looks very similar to what I had previously attempted to do (see OP) with MKAnnotationView. I haven't been able to investigate further as I'm still on iOS 17, but assuming it works as advertised, does it work because the MKAnnotationView/MapKit Framework in iOS 18 has bug fixes or for another reason?