css comment placer un icon sur une photo

.container { position: relative; }
.container img { display: block; }
.container .fa-download { position: absolute; bottom:0; left:0; }

<link href="https://use.fontawesome.com/releases/v5.7.1/css/all.css" rel="stylesheet"/>

<div class="container">
   <img src="https://placekitten.com/300/300">
   <a href="dog.png" download="new-filename"><i class="fas fa-download"></i></a>
</div>