I am right there with you on this. I recently published a Cascade story map (https://arcg.is/1WGjOa ) where I had planned to use an image for the title/cover slide instead of a text title.
As you likely found, the app won't let you just put in spaces either, it requires visible text. I ended up putting a single "." (period) with a few spaces before it to get it to line up over black text in the background image so it would satisfy the default requirement.
I was however happy to see that I could save it then go into app details and edit the title without that title overriding my "." on the maps cover.
I understand that mobile devices handle text and images differently and Esri is working to create solutions that are simple to produce while being cross platform, however It would be great to see more flexibility on this issue.
