window.onload = function() {
   var aImg = $('thumbs').getElementsByClassName('ee', 'IMG');
   for( var i=0; i < aImg.length; i++ ) {
      $(aImg[i]).addEvent('mouseover', thumbMouseover);
   }
};

// event handler functions
function thumbMouseover( e ) {
   var srcImg = this.src;
   test = srcImg.replace('imgresize.100', 'image');
   $('bigGallery').src = test;
}