If you have more than one image and you need something generic that doesn't depend on a naming convention.
HTML
<img data-other-src="big-zebra.jpg" src="small-cat.jpg">
<img data-other-src="huge-elephant.jpg" src="white-mouse.jpg">
<img data-other-src="friendly-bear.jpg" src="penguin.jpg">
JavaScript
$('img').bind('mouseenter mouseleave', function() {
$(this).attr({
src: $(this).attr('data-other-src')
, 'data-other-src': $(this).attr('src')
})
});