[Zope] Images slow rendering problem

Hi, I already posted this message several monthes ago, expecting a new Zope version would fix the problem, but... I have some images on my pages (let's say 10 - not a lot), rendered either by dtml-var or a manual <IMG> tag. When I render the page on some of my LAN boxes (especially with IE), some images doesn't load at the first time. I need to reload the page to make them appear. When I monkeyly hit the 'F5' key, some images do not show again. Argl. Anoying.... The problem is worst ever through Internet. Any clues ? Is it a Zope images rendering problem ? Many thanks, P.-J.