Custom Storyline Block "play" image for mobile users

EDIT: Researching this a bit more, it seems like some people have a thumbnail of the slide for their Storyline Blocks. Am I doing something wrong that causes me to just see a black rectangle instead of a thumbnail?

Original post:

A lot of my learners access their training on mobile devices, so I mostly use Rise. However I do sometimes utilize Storyline blocks, and when my learners view those on mobile, they are shown a black tile with a play button on it, which they need to select to start the Storyline block.

I realize this function is required by the phone's security features, but it would be really nice if I could customize the tile being shown beneath the play button.

Even if I could just make it a white tile instead of black, that would be a big improvement.

Thanks!