<a onclick="myFunc();">Спойлер</a>&nbsp;<span id="arr" class="closed"></span><br>

<div id="infoBox" style="display: none;">содержимое спойлера</div>



<script>

var myFunc = function(){

    if(infoBox.style.display === "none"){ infoBox.style.display = "block"; }else{ infoBox.style.display = "none"; }

    if(arr.className === "closed"){ arr.className = "open" }else{ arr.className = "closed"; }

}

</script>

<style>

	.closed {background-image: url(/path_to_closed.png) no-repeat;}

	.open {background-image: url(/path_to_open.png) no-repeat;}

</style>