What is the ruling on one who swore an oath of divorce upon his wife if she posted her picture, and she posted it while she was a child?

This wording is a threat of divorce. If you break your oath, you are required to offer expiation for an oath (kaffarat yameen) if you swore by Allah or by one of His attributes. As for breaking the oath, it depends on your intention. If you intended her image as an adult, then placing her image as a child does not break the oath.