fit-img-into-div.html

<!-- Fit image into resizable parent element with no height -->
<div style="height: 0; 
            padding-top: 100%; 
            position: relative; 
            display: block;">
  <img src="https://url-to-img" 
       style="position: absolute; 
              top: 50%; 
              left: 50%; 
              transform: translate(-50%, -50%); 
              max-height: 100%; 
              max-width: 100%;">
</div>