Most newsletter (& spam) images are not embedded within the message, but are stored remotely, on a web server. You will not be able to view these images if this option is turned off or if you are working offline. You need to turn on the option to allow network access when displaying complex HTML
If you are using 2001 try ....
Edit -> Preferences -> Mail and News, make sure 'Display complex HTML in messages' and 'Allow network access when displaying complex HTML' are checked.
For 2001 and X try ....
Edit>Preferences>Mail & News>View, make sure 'Display complex HTML in messages' and 'Allow network access when displaying complex HTML' are checked.
and finally In Entourage 2004, check the new Security Preferences.
hope this helps