CX3 is definitely one of the most versatile modules, but getting the CSS right for those background images can be a bit of a rabbit hole! I’ve found that if the background image is too ‘busy,’ it completely kills the readability of the event symbols.
I’ve actually been using https://solidbg.com/ to quickly strip the backgrounds off custom icons or logos before I host them for my mirror. It’s way faster than manually editing PNGs when you just want a clean, transparent symbol that doesn’t clash with the CX3 ‘glance’ view.
Are you applying the background image to the .CX3 .event container specifically, or are you trying to do a full module background? Sometimes the z-index acts up depending on the position of other modules.