You can try this mouseover event of javascript:-
<body onload="load()">
<div style="float:left;">
<button class="hover-btn" value="dragon.jpg">Dragon</button>
<button class="hover-btn" value="phoenix.jpg">Phoenix</button>
</div>
<div style="float:left;">
<img id="imgChange" src="dragon.jpg" />
</div>
<script>
function load(){
var btns = document.getElementsByClassName("hover-btn");
for(var i =0; i< btns.length; i++){
btns[i].addEventListener("mouseover", function( event ) {
document.getElementById("imgChange").src = event.target.value;
}, false);
}
}
</script>
</body>