var ImgArray = new Array();

ImgArray["11"] = new Image();
ImgArray["12"] = new Image();
ImgArray["21"] = new Image();
ImgArray["22"] = new Image();

ImgArray["11"].src = './img/ent_0101.jpg';
ImgArray["12"].src = './img/ent_0102.jpg';
ImgArray["21"].src = './img/ent_0201.jpg';
ImgArray["22"].src = './img/ent_0202.jpg';

function ImgOver(Name){
  if(Name == '1st'){ document.images[Name].src = ImgArray["12"].src; }
  if(Name == '2nd'){ document.images[Name].src = ImgArray["22"].src; }
}

function ImgOut(Name){
  if(Name == '1st'){ document.images[Name].src = ImgArray["11"].src; }
  if(Name == '2nd'){ document.images[Name].src = ImgArray["21"].src; }
}