If you are running XP with a SP2 update on it, you might have run into problems there. Installing SP2 automatically sets Outlook up to “Block Images and Other External Content in HTML email. To change this, go to Tools —> Options —> Security and remove the check from this box.
Another possibility is an overly enthusiastic anti virus or firewall. If there is a privacy setting to “disable web bugs”, it could be mistaking your content for said bugs. Simple enough to test, if you turn the option off and check results.
That’s all I could rummage up that might be giving you this grief – good luck!