Custom CSS not working to change image on hover

I am trying to have an image swap with another image on hover. 

This is my code:

#yui_3_17_2_1_1573589440099_107 {
    background-image: url(https://static1.squarespace.com/static/5c6d99d…/t/5dc91c4…/1573461074565/marks_thumbnail.png);
  background-size: 646px 470px;
}

#yui_3_17_2_1_1573589440099_107 img:hover {
  opacity: 0%;
}

When I use this within Chrome developer tools (right-click>inspect) in the style/css section, it works like a charm. But inserted into the custom CSS section of Squarespace, nothing happens.

this is my website: https://www.mrndacreative.com/

Don't use #yui

It will change if you reload the page.

Use #block_yui