Javascript image highlight (explain)

Hi I have been designing some websites, using a basic knowledge of javascript and thorough css and html. I have recently been assigned to add some simple drop down menus to another website, this in itself is simple but the website has some existing code which is both long and i dont understand. Simply i would love someone to help me convert this into jQuery and also how it works, ie how can i change it to get it to work with my drop down menus. Here it is:

<script type="text/javascript">
<!--
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开发者_运维技巧]=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_nbGroup(event, grpName) { //v6.0
  var i,img,nbArr,args=MM_nbGroup.arguments;
  if (event == "init" && args.length > 2) {
    if ((img = MM_findObj(args[2])) != null && !img.MM_init) {
      img.MM_init = true; img.MM_up = args[3]; img.MM_dn = img.src;
      if ((nbArr = document[grpName]) == null) nbArr = document[grpName] = new Array();
      nbArr[nbArr.length] = img;
      for (i=4; i < args.length-1; i+=2) if ((img = MM_findObj(args[i])) != null) {
        if (!img.MM_up) img.MM_up = img.src;
        img.src = img.MM_dn = args[i+1];
        nbArr[nbArr.length] = img;
    } }
  } else if (event == "over") {
    document.MM_nbOver = nbArr = new Array();
    for (i=1; i < args.length-1; i+=3) if ((img = MM_findObj(args[i])) != null) {
      if (!img.MM_up) img.MM_up = img.src;
      img.src = (img.MM_dn && args[i+2]) ? args[i+2] : ((args[i+1])? args[i+1] : img.MM_up);
      nbArr[nbArr.length] = img;
    }
  } else if (event == "out" ) {
    for (i=0; i < document.MM_nbOver.length; i++) {
      img = document.MM_nbOver[i]; img.src = (img.MM_dn) ? img.MM_dn : img.MM_up; }
  } else if (event == "down") {
    nbArr = document[grpName];
    if (nbArr)
      for (i=0; i < nbArr.length; i++) { img=nbArr[i]; img.src = img.MM_up; img.MM_dn = 0; }
    document[grpName] = nbArr = new Array();
    for (i=2; i < args.length-1; i+=2) if ((img = MM_findObj(args[i])) != null) {
      if (!img.MM_up) img.MM_up = img.src;
      img.src = img.MM_dn = (args[i+1])? args[i+1] : img.MM_up;
      nbArr[nbArr.length] = img;
  } }
}
//-->
</script>

Many Thanks

That looks like code left over from an old WYSIWYG editor (MM may be the acronym of MacroMedia).

By the time you de-obfuscated it and figured out what it does, you may as well have rewritten it in a readable way with jQuery.

Update

I just looked at your site, and the effect looks like it could be achieved using CSS alone (more specifically, the :hover pseudo class).

Update

I'd rather use CSS and background images, but this will work if you want to keep img elements. Just name the hover state image something sane.

Also, why does every li have its own ul parent... weird.

$('#nav li a img').each(function() {
   var originalSrc = this.src,
       hoverSrc = originalSrc.replace(/\.(gif|png|jpe?g)$/, '_hover.$1'); 

   $(this).hover(function() {
      this.src = hoverSrc;
   }, function() {
      this.src = originalSrc;
   });
});