¿Hay alguna forma de crear un modal a partir de una imagen de fondo?

El ejemplo es de w3schools:

CSS:

 .someplace { background-image: url("img_tree.gif"); }

html:

 <div class="someplace"></div> <div id="myModal" class="modal"> <span class="close">&times;</span> <img class="modal-content" id="img01"> <div id="caption"></div> </div> <script> // Get the modal var modal = document.getElementById("myModal"); // Get the image and insert it inside the modal - use its "alt" text as a caption var img = document.getElementById("myImg"); var modalImg = document.getElementById("img01"); var captionText = document.getElementById("caption"); img.onclick = function(){ modal.style.display = "block";

??????????????? modalImg.src = ??????????

 captionText.innerHTML = this.alt; } // Get the <span> element that closes the modal var span = document.getElementsByClassName("close")[0]; // When the user clicks on <span> (x), close the modal span.onclick = function() { modal.style.display = "none"; } </script>

Gracias