Recently, this is a problem on my iPhone and sometimes my iPad. IOS 15.4.1. On my iPad 15.3.1 but just checking, looks ok now on iPad but not iPhone.
This has been an ongoing issue from Aladin Lite, the third party library used to display the background imagery. I’m planning to change this so we can have a more reliable experience.
As a workaround, closing all open Telescopius tabs and clearing your browser cache usually gets the images going again. I hope this helps!