Altra soluzione:
1. parte Javascript
codice:
normeOver = new Image();		
normeOver.src = "images/norme_over.jpg";	
norme = new Image();		
norme.src = "images/norme_to_select.jpg";

function imgOn(imgName)
{   if (document.images)
	{  document.getElementById(imgName).src = eval(imgName + "Over.src");
	}
}

function imgOff(imgName)
{	if (document.images)
	{document.getElementById(imgName).src = eval(imgName + ".src");
	}
}
2. parte codice HTML
codice:
[img]images/norme_to_select.jpg[/img]