assuming the pp_photos table is all that you have to change an update statement should do the trick. there may be other tables, but I am not sure. something like
SET pp_photos.user = 'username'
WHERE pp_photos.user = 'oldusername';
Its interesting they have the user id and user name in the table. I am assuming it is for performance reasons not having to do a join to get the username from the pp_users table. only a guess though.