I am an idiot, BTW. THe URL I used included a query string. The database query obviously timed out after half an hour or so, after which the URL was no longer valid. I should have realized that before posting.
But I'm not that far off: just about the only reason I can think of for anyone to write a result page that relied on the query string that way is hide the location of the graphic. The reason to do that is to stop people piggybacking on your badnwidth by referring directly to the graphic.