There is a round about way to see the photos for items that are part of the child database within a relational database schema within ArcGIS Online. It is not exactly within the pop-up window, but it would allow user to see there photos.
For instance, we do a lot of endangered Bat surveys and one of the database schema that I utilize within the AGO environment is a point that marks that location of the survey and then a related table that contains the vital information for the bats that are captured during the survey. This is a table that is part of a Global ID relationship to the point feature class.
When you click on the point location; within the pop-up window you have the ability to "Show Related Records" and that can be found at the bottom of the pop-up window.
After clicking on this link you have the option to choose which related item you would want to see. Meaning, if you have 3, 4, 5, or how ever many related records, then you will have the option to view them within the table viewer (it appears at the bottom of the screen). Then the user would have to click on the correct record, scroll all the way to the right and then click on hyperlink "show" within the "Photos and Files" data fields to view the photos.
It's not exactly the pop-up window scenario that you are looking for, but it would help for the time being.
One other option would be to generate an "Operations Dashboard" that would display each of the photos within your related table. I had taken that route also, because you can setup a map view and then a "Widget" that would allow you to display the photos that are part of a related feature.
Hope this helps in some way.
Mike