It happens that the iCloud contact photos in my iPad (iOS 9.3 public beta) do not update after I changed it in iPhone/iMac.
This could be solved by choosing one of the contacts that did not update itself, tap on "edit contact", then "edit photo", then "delete photo". After the photo is deleted, it will automatically update itself.
This could be solved by choosing one of the contacts that did not update itself, tap on "edit contact", then "edit photo", then "delete photo". After the photo is deleted, it will automatically update itself.