How to put a prompt when there is no image during popup
	
	
		Hello!  I have a the following code at the head
===
function popUp(popurl){
var winpops=window.open("","LargeImage","location=no,m  enubar=no,status=no,directories=no,scrollbars=no,t  oolbar=no,width=350,height=500");
if (popurl != null) {winpops.document.write("<center><img src= '"+popurl+"' >"); }
else {
winpops.document.write("<BR><BR><BR>");
winpops.document.write("<br><center><img src= '"+popurl+"' alt='No Image Available at this Time' >"); 
}
winpops.document.write("<BR><BR><BR>");
winpops.document.write("<input type='button' value=' Close Window ' onclick='javascript:window.close()'>"); 
}
===
 
and at the body
<A HREF="javascript:popUp('computer.jpg')"><img src="computer.jpg"></a></TD>
How do I put a message that tells if that image is not available? Thanks