[Zope] Missing pictures

Hello, I have notified that every time on my web page one picture is missing (with IE and Netscape). When I refresh the page there is another picture missing and the first is OK. After another refresh, the first is missing and the second is OK. And so on... I use Apache and Zope 2.1.6 under Linux. In the Apache log, I have notified that http code is always 304 but when pictures are missing in web page the transferred size is '-' and no '0'. What's goes wrong ?