The answer, in a word, is CRON. [Hey, you asked, right?? :) ]

On servers like the one we use, there is a service called CRON that can run certain programs as many times as we like each day/hour/minute/month/you-get-the-idea. (And yes, it comes from the Greek word chronos.) Since it takes up a lot of resources to run all of the programs that we ask of it - and we do, admittedly, ask it to run absolutely everything in one fell swoop - CRON runs once a day. When it runs, it does a lot of housekeeping, re-indexing the site search and, oh yeah, getting some of the new user images into the profiles.

It's mildly annoying, I know, but it gives us a chance to catch some untoward stuff before it reaches the public (no comment on how often that's happened...) and keeps the site quicker during the day because it's not running mini-CRONs all the time.