var mapID = "m_mapimg"; 　// mapのidを入力
var mapImgID = "mapimg"; // mapに使う画像のIDを入力
var btnNames = ["om1","om2"]; // ボタンの名前を入力

var btns = new Array();
var offImg = new Image();
var smapImgs = new Array();

window.onload = swapMapImg;

function swapMapImg(){
  btns = document.getElementById(mapID).getElementsByTagName("area");
  preLoadImg();
  setEvent();
}

function preLoadImg(){
  offImg.src = document.getElementById(mapImgID).src;
  var temp = offImg.src;
  var tempA,tempB;
for(var i=0;i<btns.length;i++){
    smapImgs[i] = new Image();
    tempA = temp;		
    tempB = '-' + btnNames[i] + '.gif'
    smapImgs[i].src = tempA.replace('.gif',tempB);
  }
}

function setEvent(){
  for(var i=0;i<btns.length;i++){
      btns[i].onmouseover  = new Function("document.getElementById(mapImgID).src=\"" + smapImgs[i].src + "\";");
      btns[i].onmouseout = new Function("document.getElementById(mapImgID).src=\"" + offImg.src + "\";");
  }
}