Porque me da error? si se supone que todo lo ejecute bien.. :/
<button type="button" onclick="llamada()">Set the background image of div
</button> <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"/>
function llamada(){
document.getElementById("bgc").style.backgroundImage = "url('assets/h1.png')";
}