The functionality doesn't seem to exist to display image attachments in Gallery Mode for a web map pop up and subsequently for the Dashboard that reads the map pop up when using enterprise published services. I get the message: "This layer only supports list view". We have a lot of data published through ArcServer services and it's very limiting that the Gallery View is only available for Esri hosted services. I did not see a Map Viewer ideas board, and we ultimately are trying to display the image attachments in a Dashboard.