Set a default value for a jsonb property in PostgreSQL

Let’s say that I have a jsonb column with documents like:

{
    "lead": "The lead",
    "video": "An URL to the video"
}

In some cases I also need to store the name of a picture that’s uploaded to the server by the user in the JSON. Obviously I don’t want clashes on the filenames so I wonder if it’s possible to use uuid_generate_v4() as default value:

insert into document (title, content) values (
    "The title of the document",
    '{
        "lead": "The lead",
        "video": "An URL to the video",
        "picture": ":?" -- Here I need the value returned by uuid_generate_v4()
    }'
);

Is it possible or should I get the uuid in the application before inserting the data in the database?

Thanks beforehand.