Thanks for your reply.
The overall performance of the website has improved tremendously since installing, tweaking, and re-tweaking W3 Total Cache, with post pages now usually loading within about 2 seconds (as measured and recorded by pingdom.com), when not logged in, compared to 5-8 seconds when logged in. [WP 3.2.1; Thesis 1.82; hosted on a shared server at a major hosting company.
What drew attention to the old post pages with .jpg images is that now those images load noticeably slower than their respective pages. Before W3, the page and image appeared more or less at once. Now, on old pages with old jpg images, the image loading trails behind the page. New pages with jpeg files (with mode_expiry set by W3), the image doesn't trail behind.
All that is via a browser where the reader is not logged in to the website and there's no browser caching effect from prior visit affecting the pages differently for the two situations.
The question remains: is there a way to have W3 Total Cache also apply the defined mode_expiry rules to jpg files?
Thank you again!