JSFiddle - React, Tailwind, and code Playground

HTML

<div id="section0">
Example test. Lorem ipsum dolor sit amet.
  <div id="bg-opacity"></div>
</div>

CSS

#section0{
}

#section0 #bg-opacity{
     position: absolute;
     top: 0;
     left: 0;
     z-index: -1;
     opacity: 0;
     transition: opacity 2s; /* Define the duration of the opacity animation */
     height: 500px;
     width: 500px;
     background-image: url("https://media.giphy.com/media/tIeCLkB8geYtW/giphy.gif");
}

JavaScript

setTimeout(function(){ 
  $("#bg-opacity").css({
       "opacity" : 1,
       "background-image": "url(https://media.giphy.com/media/tIeCLkB8geYtW/giphy.gif)"
  });
}, 300);