When images are in the CSS, are you supposed to put any tag at all?
I just read this:
No, because a background image is only a decorative element which should not have any semantic meaning. HTML is for semantics and meaning, CSS is just for visual appearances. If the image is so important that it should have a fallback alt text, make it an HTML element.
No alt tags are required or allowed in background images
That's right CSS background images have no relation to the html <img>
This is all being discussed in this other thread
Please reserve this thread for your Understanding the use of aria-hidden