In Firefox, you can open Tools - Web Developer - Network. Then open or refresh the page you're interested in that same tab. It will list all the requests sent to fetch the data to display for that page, including the images. When you select one of the entries, it will display details for that request & response, including the headers.

For the images on this page, I see response headers including "x-cache: Miss from cloudfront" or "x-cache: Hit from cloudfront". I'm not sure if that is related to the Dreamwidth image proxy or something else.