This trick is nice, but can quickly be overused. Remember that people on mobile devices will not have the same experience with image rollovers as those on a desktop. So if the image rollover is critical in communicating an idea and you think or know people will be visiting on a phone, choose another method.
You will need to do this in HTML mode:
<a href="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>