On your game page, click “Edit Theme”. If you scroll down there is an option for screenshots. The default should be “Auto” which hides the screenshots if there is an HTML playable version. You can change that to “Sidebar” to force them to show up no matter what.

I guess the idea behind this is, if you can play the game straightaway, there is no much point looking at the screenshots. But at the end of the day, you are the creator, so you have the option to override that.