that would tell me that you have some anti hotlinking rules in your .htaccess file.
I'm wondering though what the referrer looks like. Norton used to set it to 'Blocked by Norton', but, for a while was sending a blank referrer, and for a while were sending XXXX:XXXXXXXXXXXXX.
Another forum I follow was talking about the issue about blocking bad versus allowing good, since, most .htaccess rules will also block google from hotlinking in the
http://images.google.com section.