Try this it`s so easy and the shortest one, just change the Image’s URL:
<a href="https://stackoverflow.com/questions/9400883/TARGET URL GOES HERE">
<img src="URL OF FIRST IMAGE GOES HERE"
onmouseover="this.src="URL OF SECOND IMAGE GOES HERE";"
onmouseout="this.src="URL OF FIRST IMAGE GOES HERE";">
</a>