Hi Josie- FYI, your foo.png is showing a sample from the new StoryMap Builder, not the Class template builder. I have seen this too, no image displays in the pop-up when presented in the new StoryMaps, thought my attributes do display.
I do not know the answer as to why web map pop-ups in the new builder do not show the pop-up media, perhaps that functionality is not set up and they are waiting for the full release of the new Map Viewer? Hope you get more clarity for this post!
Thanks-
Kate