<script type="text/javascript">
<!--
function MM_swapImgRestore() { //v3.0
 var i,x,a=document.MM_sr; for(i=0;a&&i<a.length&&(x=a[i])&&x.oSrc;i++) x.src=x.oSrc;
}
function MM_preloadImages() { //v3.0
 var d=document; if(d.images){ if(!d.MM_p) d.MM_p=new Array();
 var i,j=d.MM_p.length,a=MM_preloadImages.arguments; for(i=0; i<a.length; i++)
 if (a[i].indexOf("#")!=0){ d.MM_p[j]=new Image; d.MM_p[j++].src=a[i];}}
}

function MM_findObj(n, d) { //v4.01
 var p,i,x; if(!d) d=document; if((p=n.indexOf("?"))>0&&parent.frames.length) {
 d=parent.frames[n.substring(p+1)].document; n=n.substring(0,p);}
 if(!(x=d[n])&&d.all) x=d.all[n]; for (i=0;!x&&i<d.forms.length;i++) x=d.forms[i][n];
 for(i=0;!x&&d.layers&&i<d.layers.length;i++) x=MM_findObj(n,d.layers[i].document);
 if(!x && d.getElementById) x=d.getElementById(n); return x;
}

function MM_swapImage() { //v3.0
 var i,j=0,x,a=MM_swapImage.arguments; document.MM_sr=new Array; for(i=0;i<(a.length-2);i+=3)
 if ((x=MM_findObj(a[i]))!=null){document.MM_sr[j++]=x; if(!x.oSrc) x.oSrc=x.src; x.src=a[i+2];}
}
//-->
</script>

// html code
<a href="#" onmouseout="MM_swapImgRestore()" onmouseover="MM_swapImage('Image1','','2.png',1)"><img src="1.png" name="Image1" width="155" height="71" border="0" id="Image1" /></a>