Another option is to add a link to somewhere that does support multiple photos per location. How you implement this depends on the story map template you plan to use. For example, using the Map Tour you could add a hyperlink to Flickr photo set, or some other website that enabled multiple photo display. For the templates that expose the web map pop-ups, these links can be configured directly in the pop-up window. In the past I've used JAlbum to support multiple photos at one location. That's not an endorsement, it's just a handy tool that I have used to accomplish what you want, and there are lots of similar alternatives to chose from.
