Attempting to set user profile photo via API is failing

I am attempting to set the user profile photo via the API.  I am able to add and update users, but I am having issues with adding the photo.

I am getting the following error returned in the headers:

x-zendesk-api-warn, val={:allowed_parameters=>{:controller=>"users", :action=>"update", :unpermitted_keys=>["user.photo.content_url"], :invalid_values=>[]}}

  I am sending the following JSON:

{
  "user": {
    "photo": {
      "content_url": "https://apex.cct.rpi.edu/apex/simon_apex.display_blob?URL_KEY=GHu5iY48fMQC6ExnuardXOLNd"
    },
    "id": 7412860633620,
    "external_id": "91000000",
    "name": "Jon Finke",
    "email": "finkej@rpi.edu",
    "verified": true,
    "user_fields": {
      "display_name": "Jon Finke",
      "first_name": "Jon",
      "last_name": "Finke",
      "rcs_id": "finkej",
      "portfolio": "Division of the Chief Information Officer (DotCIO)",
      "department": "Database Administration"
    }
  }
}