Yes, this seems to work*. O_o

See the Pen
Image with Background Image (yes you read that right) by Tom (@webrocker)
on CodePen.

I wondered if I could use a background image for a transparent png I used in a recent blog post, and decided to give it a try and not making a wrapper around the img, but add the "background-image" style directly to the img tag.
Imagine how surprised I was to see that this works, (*at least on Firefox, Chrome and Safari on macOS 10.15.6 and Safari on iOS 13.5.1).

 .fancyimg {
    padding: 5vh 15vw;
    background-image: url(http://placekitten.com/600);
    background-size: cover;
  }

Here's a screenshot of the result (Firefox 79.0, macOS 10.15.6):
An image of a cat, and in the background another image of a cat

This may open a whole new bunch of opportunities for example if the img src couldn't be loaded and a fallback/placeholder should be displayed.