While creating LastPass we wanted to show an overlay on the page when you autologin to a site. We ran into an issue though: if we used a image on that overlay, the image would leak the referring URL -- a privacy leak that we wanted to avoid.
In Firefox, there is a relatively straight forward and elegant solution: use an inline base64 encoded image. This method is covered here: http://www.websiteoptimization.com/speed/tweak/inline-images/
Granted this is a very small image (our logo), and we probably wouldn't do it if we needed a very large image, but it accomplishes the goal while protecting your privacy which makes us happy.