Is your feature request related to a problem? Please describe.
right now Cover Block uses background-image which is not ideal solution (one image size to all devices)

Describe the solution you'd like
instead of using background-image it could use <img> element with srcset and additional styles (example, .wp-block-cover__image is <img> element):
.wp-block-cover { position: relative; }
.wp-block-cover__image { position: absolute; top: 0; left: 0; right: 0; bottom: 0; width: 100%; height: 100%; object-fit: cover; object-position: 50% 50%; }
possibly with option to alter sizes attribute

Describe alternatives you've considered
Removing Cover block and replacing it with custom made one.