Hi Olivia, it sounds like you are not referencing your images via a URL to a publicly accessible web location. Your images need to be on the web at a location so they can be accessed publicly via a URL. When you upload a shapefile CSV containing these URLs into an ArcGIS.com map viewer, the default popup will just show you a 'more info' link, like you describe. But when you display your web map in the Shortlist tmeplate the photos will be displayed correctly if the shapefile or CSV file you added into your web map uses the expected set of shortlist attributes.
If it isn't working still, could you share your web map publicly and put the URL here so we can try it out ourselves and see what is wrong?