We're hosting the photos on S3 currently, only around 30K photos currently and the cost of S3 is dependent on how frequently they are accessed (how often users load a specific photo). If I recall correctly our S3 bill last month was around 3 dollars but we don't have very many users. I'm sure its not scalable if we had tons of photos and tons of users but I think I'm just crossing that bridge if I ever get there

Better question is what is the source and license of those photos? :)