var images = new Image();
images.src = "./b.gif";
function changeImage( flag ) {
if ( flag ) {
window.document.image1.src = "./b.gif";
} else {
window.document.image1.src = "./a.gif";
}
}
<A href="xxx.html" onMouseOver="changeImage(true);" onMouseOut="changeImage(false);"> <IMG src="./a.gif" name="image1" border="0" width="22" height="22"> </A>