Obviously all the images are going to be fitted to the grid in our game container so, instead of typing height:100% for every image, we can specify it just once as shown below. So here is what we have so far:

index.html
<body>
  <div id='game'>
    <img src='images/scenes/3.jpg' id='scene'>
    <img src='images/shapes/blue.png' id='blue'>
  </div>
</body>
styles.css
img {
  height: 100%;
}
#game {
  width: 120vmin;
  height: 90vmin;
  display: grid;
  grid-template-rows: repeat(10,10%);
  grid-template-columns: repeat(10,10%);
}
#scene {
  grid-row: 1 / span 10;
  grid-column: 1 / span 10;
}
#blue {
  grid-row: 8 / span 3;
  grid-column: 6 / span 2;
}