var imageShow=function(){
  var thumbs = $('thumbs')
  var bigs = $('bigs')
  thumbs= thumbs.getElementsByClassName('img')
  bigs= bigs.getElementsByClassName('img')
  num=bigs.length
  for(var i=0; i<num;i++){
    if(i>0) bigs[i].style.display='none'
    $(thumbs[i]).onclick=function(){
      for(var j=0; j<num;j++){
        bigs[j].style.display='none'
      }
      $('big'+this.id).style.display='block'
      
    }
  } 
}

Event.observe(window, 'load', imageShow.bind(this) )