If you want to add to marker description an image, which will redirect you to some link by click, you can use this code:
<a href="https://supsystic.com/" target="_blank" style="background-image: url(https://supsystic.com/wp-content/uploads/2018/07/Facebook-share-button.png);background-position: center bottom; background-repeat: no-repeat; display: block; width: 100%; height: 50px; background-size: contain;"></a>
Map example: