From the article: "For performance, each of our image servers cache the source, and any resize, locally to disk. Since images are never updated (only created)

From the article: "For performance, each of our image servers cache the source, and any resize, locally to disk. Since images are never updated (only created), and get a unique ID for each one, we don’t have to worry about cache invalidation, only expiration. We can then write a simple script to remove images from this disk cache with files of an access time greater than a certain threshold (say 30 days). That way, if we change from one size thumbnail to another, eventually the old thumbnail sizes will get purged."