I have a cousin who buys pictures with some identification, and then locates descendants. He mails the pictures as a gift. They have been thrilled to find pictures of ancestors they have never seen. This is a dawning process.
There was a Jerry Lewis movie where he proved he was an heir to a fortune by shaving the beard of a portrait of an ancestor.
From the mere outside appearance, the identifiers are the ears, mostly, and the nose, secondarily (investigation studies that go way back). With special equipment, it’s the irises and retinas (newer stuff).