When "images are coming in at the rate of thousands per day," how many of those images are being viewed? And of those images, how many images are being viewed more than once? Will the demand for images continue to remain high, or do the odds-of-viewing drop as the image becomes older?

One simple strategy is to see if a reduced version of the image already exists. (Look in a "thumbnails" sub-directory, say...) if it does, display it. Otherwise, create a reduced image first and then display it. (Future requests for the same image will only need to display it.)

If you expect that most of the images will be viewed eventually, then you could have a Perl program running continuously in the background, crawling through the directories looking for images that have not yet been reduced and reducing them. This process runs all day and all night, happily slurping-up excess CPU cycles at times when the computer has nothing better to do. (e.g. “nice background_shrinker.pl &”)