"IMPORTANT: This FAQ contains information about modifying the registry. Before you modify the registry, make sure to back it up and make sure that you understand how to restore the registry if a problem occurs.
Use the instructions here
to create a new registry key. Note you must have SP1 in order for this feature to work. This will cause all images received in future Outlook emails to become attachments (even embeded ones).
To turn off this feature, just go back to the same registry key you just created HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Options\Mail and change the key value from 1 to 0.
To simplify toggling back and forth, export the newly created branch from regeit, and named it OutlookReadAsPlainTextOn.reg. Then edit that file changing the 1 to a 0 as mentinoned above and saved that file as OutlookReadAsPlainTextOff.reg. Toggle at will by double clicking the appropriate .reg file and accepting the merge or change.