I'm really interested in this deal. It's always been a bit of an enigma to me how imgur makes money, especially since they have been bootstrapping. I wonder if the numbers looked good to a16z, or if they saw the incredible growth potential of having a large interest in the youtube of images (as frade33 characterized it).
A high traffic image sharing site can actually be pretty cheap to run. Cloudflare gives you an unlimited bandwidth CDN and incoming bandwidth to S3 is free (uploads can be done directly from the browser).