hey Haki,
I am sorry for the delay of this reply,
Yes, that was correct, but I believe your site user also unable to tell you how much photo they will upload in your site social network, because this might be unlimited value.
What my suggestion is consult with your current webhosting provider and Amazon S3 price is it increase your server storage space or upload to Amazon is better.
To be honestly, I am not really sure what kind of pricing you should look for, what i know is they will charge you per month base on your Data Transfer/Get Requests/Storage.