Those guys at Facebook sure are smart. Here is an online presentation titled Needle in a Haystack: Efficient Storage of Billions of Photos.
Jason Sobel, the manager of infrastructure engineering at Facebook, … explains how Facebook efficiently stores ~6.5 billion images, in 4 or 5 sizes each, totaling ~30 billion files, and a total of 540 TB and serving 475,000 images per second at peak.