OWA Images not showing

Hello,
I have recently installed a hardware router (Netgear FVS318). Now, when I try to access Exchange 2000 on a win 2k server through Outlook Web Access, I get to log in OK, but I only get image placeholders, instead of images and nothing else.Microsoft's support site mentioned folder permissions and anonomyous logons, and a fix which was supposed to address the problem I had. I did this. All that changed was replacement of ISA Server 2000 software with the router. Microsoft's Web site also mentioned a whole heap of ports to open up to get OWA working but I am hesitant to do this. I could change clients to POP3 (leaving messages on the server)???