Hey!
I'm running into the same issue with my calendar. I used the code above and it does show the image now on mobile, but now my desktop site doesn't show images. Also with mobile it doesn't show the entire image, just a small corner of it.
edit: So it works on the squarespace website itself when you swap between desktop and mobile view. But once you actually pull it up on a phone it still shows the dark imageless tiles for those dates.
squarespace site view: https://gyazo.com/56e66703e9d21cc4f336f9a5a9291bfa
actual mobile view: https://gyazo.com/2f7fb191565f145fce6d96fbba6d5117
link to site: https://www.wiz4rdssleev3s.com/events