<button type="button" onclick="llamada()">Set the background image of div</button>
<div>
     <div id=bgc" class="c1" style="width:600px;background-image: url('assets/b1.png'); background-repeat: no-repeat;">
     <img class="caobj" src="assets/d5.png" width="400" height=300"/>
     </div>
</div>
 

 function llamada(){
          document.getElementById("bgc").style.backgroundImage = "url('assets/h1.png')";
    }