Prova così:
codice:
<HTML><BODY>
<DIV align="center">
<FORM action="swap.html" method="post">
<INPUT id="a" type="button" value="AAAAA" onMouseOver="do_it();" onMouseOut="Restore();"></FORM>



</DIV>
<SCRIPT language="Javascript1.2">
<!--
function do_it()
{
var wb=document.getElementById("b").width;
var wc=document.getElementById("c").width;
document.getElementById("b").src="http://www4.nau.edu/bilby/Images/letter_B.gif";
document.getElementById("c").src="http://www.julen.net/cfp/alphabet/digital/img/c.gif";
document.getElementById("b").width=wb;
document.getElementById("c").width=wc;
}

function Restore()
{
var wb=document.getElementById("b").width;
document.getElementById("b").src="http://www.designsinstitches.net/Design%20Stitchouts/ABC%20Applique%20Quilt/Letter%20B.jpg";
document.getElementById("b").width=wb;
}
//-->
</SCRIPT></BODY></HTML>