Can't get image link to click above image

I was wondering how to place a link as an image above another image and make it work. I tried doing a CSS carousel and cannot get most of the image links to be clickable, unless I get the image links completely off the underlying image. I tried using z-index in an attempt to get the span a img element above the other img but that doesn’t work. Any insight appreciated.

Here is my link on CodePen: