I subscribe to several rss feeds to track incoming links, on sites like Technorati, Talk Digger, Google Blog Search and Daypop. Sometimes these feeds will contain one of my posts. When I view these posts in NetNewsWire, the linked images do not appear.
From looking at my logs, I found out why. I have a mod_rewrite rule that denies http requests for images when a referrer is specified, and it is not from this domain. It appears that NetNewsWire uses the domain that the rss feed originated from as the referrer for all links. I actually think that this is a good thing, as content providers can still tell where their content is linked from.