isamap = new Object();
isamap[0] = "_df"
isamap[1] = "_ov"
isamap[2] = "_ot"
isamap[3] = "_dn"

function isimgact(id, act)
{
	if(document.images) document.images[id].src = eval( "isimages." + id + isamap[act] + ".src");
}

if (document.images) { // ensure browser can do JavaScript rollovers.
isimages = new Object();
isimages.main_df = new Image();
isimages.main_df.src = "/images/main.jpg";

isimages.main_ov = new Image();
isimages.main_ov.src = "/images/mainov.jpg";

isimages.articles_df = new Image();
isimages.articles_df.src = "/images/articles.jpg";

isimages.articles_ov = new Image();
isimages.articles_ov.src = "/images/articlesov.jpg";

isimages.board_df = new Image();
isimages.board_df.src = "/images/board.jpg";

isimages.board_ov = new Image();
isimages.board_ov.src = "/images/boardov.jpg";

isimages.joinmmta_df = new Image();
isimages.joinmmta_df.src = "/images/joinmmta.jpg";

isimages.joinmmta_ov = new Image();
isimages.joinmmta_ov.src = "/images/joinmmtaov.jpg";

isimages.member_df = new Image();
isimages.member_df.src = "/images/member.jpg";

isimages.member_ov = new Image();
isimages.member_ov.src = "/images/memberov.jpg";

isimages.calendar_df = new Image();
isimages.calendar_df.src = "/images/calendar.jpg";

isimages.calendar_ov = new Image();
isimages.calendar_ov.src = "/images/calendarov.jpg";

isimages.jobopps_df = new Image();
isimages.jobopps_df.src = "/images/jobopps.jpg";

isimages.jobopps_ov = new Image();
isimages.jobopps_ov.src = "/images/joboppsov.jpg";

isimages.contact_df = new Image();
isimages.contact_df.src = "/images/contact.jpg";

isimages.contact_ov = new Image();
isimages.contact_ov.src = "/images/contactov.jpg";

}
<!--
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_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_findObj(n, d) { //v4.0
  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 && document.getElementById) x=document.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];}
}
//-->