it does not literally "become property", but in their terms, which you have agreed, you knowingly give them the eternal and irrevocable right (license) to use them free of charge and even to provide this license to other entities (even for a fee) it differs from "they become their property" only in that they can't stop you from using and distributing those photos in other ways (which you can't forbid them anymore) it's the same in that you can never take anyone on those photos again provide an exclusive license, they also can't use them in advertising or modify them if they used them in some derogatory way (especially your selfies), you could probably sue them