Add this code to the website or to the Joomla page:
[code]
<p><img title="text for small pop-up" height="129" alt="alternative text" width="210"
onmouseover="this.src='/images/stories/2st-image.jpg';"
onmouseout="this.src='/images/stories/1st-image.jpg';" src="/images/stories/1st-image.jpg" /></p>[/code]