function findimg(){
var imgs,i;
// Loop through all images, check if they contain the class roll
imgs=document.getElementsByTagName('img');
for(i=0;i<imgs.length;i++){
	if(/roll/.test(imgs[i].className)){
		// add the function roll to the parent Element of the image
		imgs[i].parentNode.onmouseover=function(){roll(this);};
		imgs[i].parentNode.onmouseout=function(){roll(this);};
		imgs[i].parentNode.onfocus=function(){roll(this);};
		imgs[i].parentNode.onblur=function(){roll(this);};
		}
	}
}

function roll(o){
var i,isnode,src,ftype,newsrc,nownode;
// loop through all childNodes
for (i=0;i<o.childNodes.length;i++){
	nownode=o.childNodes[i];
	// if the node is an element and an IMG set the variable and exit the loop
	if(nownode.nodeType==1 && /img/i.test(nownode.nodeName)){
		isnode=i;
		break;
	}
}
// check src and do the rollover
src = o.childNodes[isnode].src;
ftype = src.substring(src.lastIndexOf('.'), src.length);
if(/_on/.test(src)){
	newsrc = src.replace('_on','');
	}
else{newsrc = src.replace(ftype, '_on'+ftype);}
o.childNodes[isnode].src=newsrc;
}




function addOnLoad(nuevoOnLoad) {
  var prevOnload = window.onload;
  if (typeof window.onload != 'function') {
    window.onload = nuevoOnLoad;
  }
  else {
    window.onload = function() {
    prevOnload();
    nuevoOnLoad();
    }
  }
}


addOnLoad(findimg);      //esta función estaría ya definida
/*addOnLoad(  //para pasar parámetros usar función anónima
  function() {
  alert('hola 11');
  alert('hola 12');
  }
);*/