The fairly typical method of documenting an item to a date in time would be to wrap the item in an acid free paper and/or some other archival medium for long term storeage, seal that item, i.e. picture, in an envelope and mail it to yourself via certified / registered U.S.Mail.
You don't actually have to have the Post Office send it since it is being mailed by you to you, they ( the PO ) will take the sealed envelope, date stamp it, you will pay whatever postal charges there are and then they will "deliver" it to you by handing it back.
Don't open the envelope. write on the outside ( somewhere that does not disturb the registration and stampings) what it contains and put it in your strong box or safety deposit box. If, in the future, this becomes a problem, get yourself an attorney and give him the sealed UNOPENED envelope with the date stamps INTACT so that he can establish the chain of custody and he will open the envelope if necessary. Until you need it, keep the picture secured away.