So if I understand
z-index:-1 // on the image tag/card
z-index:100 // on the div above
z-index only works on positioned elements (position:absolute, position:relative, or position:fixed).
Edit by TylerH:
There is also
position: sticky; that is supported in Firefox, is prefixed in Safari, worked for a time in older versions of Chrome under a custom flag, and is under consideration by Microsoft to add to their Edge browser.