I don't have an answer, unfortunately, but I'm having a broadly similar problem (at least, I think so).
Having said that, I'm not part of an organisation and most of the messages I receive that contain graphics come from commercial major companies. For some reason, most of these have started showing the red-crossed place-holders where graphics should be (although, bizarrely, some messages still show graphics correctly). This has only started happening over the last few days and I'm not aware of any changes I have made to the system that could have produced this result.
This problem has started happening even for companies where I had previously received e-mail graphics with no problems and even if the sender is on the "Safe Senders" or "Trusted Zones" list. If the system has shown a message saying that pictures have not been downloaded for my protection, clicking the "Download them anyway" button has failed to produce the missing graphics.
I've tried temporarily disabling my firewall, anti-virus and anti-spam software, as well as changing the security settings in Outlook to "Download pictures". Nothing has worked.
I am also using Outlook 2003, but not Exchange Server. I just have a straight DSL connection through the telephone line to my ISP, from where I collect my e-mail (POP3). I don't know whether it's also affecting outgoing mail, as I don't send out much containing graphics.
In case you need to know, my OS is Windows XP Professional (SP2), browser is Firefox 2 and firewall etc. is Kaspersky Internet Security 7.
I shall be very grateful for any guidance you can give, because I currently have no idea whether this problem has been caused by Outlook, Windows, Firefox, Kaspersky, my ISP or something else!