The technical reason for them not being cached is that the URL of the image changes on every page. Your browser, and any cache software, such as Squid checks for cache hits by checking for an exact URL match. A change of 1 character in the URL is interpreted as a miss and forces a re-fetch. The URL is changed intentionally so that many different ads can appear on the same page, and also to increase the number of ads you can see, which should be about one per page viewed.

PerlMonks banner advertisments are regular <IMG>s, but the source appears to have random content appended to the end. Here's a quick sample of the URLs for the banner ad which I compiled just clicking around:

In reply to Re: Caching of banners by tadman
in thread Caching of banners by stefan k

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