We've been noticing heavy traffic from some users/some browsers. They request and re-request the same image files.

I believe that the issue lies in the incorrect "Expires:" header returned.
When requesting e.g. yellow.png the headers returned are e.g:
Date: Wed, 13 Feb 2013 17:39:46 GMT
Expires: Thu, 13 Feb 2014 17:39:46 GMT

You'll notice that the day of the week changes, but the actual date and time is the same for the Date and the Expires.

Some browsers seem to cache this OK, others seem to ignore the Expires and re-request the image.