When I tested I was using webm files. I think you need both, one for mobile deployment and the other for desktop deployment, similar to how you need two of each sound effect in different formats for the same reason.
The still images should work fine if they're png files, but it's possible that trying to use them at the same time as the mp4 was causing some kind of hiccup where it was always trying to load the mp4 and never getting past that.
Check the console (f12) and see if there are any errors being reported there.