We could do daily CheckUsage for images and count on a day-by-day basis. Or just once a week. Or once a month. These will average out over time.
Err... don't we have logs of the access to the image files themselves (including thumbnails)? It would be MUCH simpler to analyse those than to look at page views and then at image usage and then infer how much an image is loaded...
Basically, there are two interesting figures: 1) "On how many pages is a given image used?" (maybe also "on how many different wikis") 2) "How often is a given image loaded?"
What has been discussed so far is "how often are pages that use the image loaded", which seems to be besides the point...
-- Daniel