Where did you add the screenshots? They are not meant to be part of the downloadable files …
When you edit the game’s page, you have the option to add screenshots and add a video URL there.
To add to the mystery, this only seems to happen on desktop browsers (e.g. Chrome, Firefox, Edge, IE, etc.). It works fine in my iPhone browser or when using the Mobile RWD Tester extension in Chrome to emulate a phone or tablet. Very odd.
I wonder if this is related?