As far as I'm aware there is no way to tell Outlook / OE of saying

"Open HTML emails, if I ask you to on a per-message basis, but don't load images from external sites because I like my privacy"
...which is where a good proxy/personal firewall would come into play. My company requires me to use Outlook for e-mail at work, so I use the many-years-old-but-still-the-best AtGuard to permit Outlook to connect to my POP mail server on port 110, and block all other outbound connections from Outlook. Allows you to preview the HTML message, but stops web bugs dead in their tracks.