Hiya! I've got a wonderful guide for CSS Neat things here;
CSS FAQ, Help and Neat Things [+IMGs]

But, so you don't have to go searching - here's this. I put in your linked image so all you have to do is copy/paste.

img[src$="default.png"] {
content: url("https://i.imgur.com/B5pWSiB.png"); }


The first bit controls the background of the Featured Lion.

This second bit controls what displays if you have no Featured Lion set. You CAN just keep this out if you don't want a different image to display when no lion is set. The image displayed will default to the code above.. ^^

.lionImage > img {
content: url("https://i.imgur.com/DisplayNoneImageOverride.png") !important; }


Image Example:
First Code Snip:


Second Code Snip: