How to link migrated image file to the user profile picture field? I used the following migration file to migrate user pictures from a Drupal 7 site installation to Drupal 8:


  migrate_plus.migration.drupal7_image.yml


id: drupal7_image
label: Images
migration_group: dhm_migrate_taxonomy
migration_tags:
  - Drupal 7
source:
  plugin: d7_file
  constants:
    source_base_path: '/var/www/html/migrate_training_7/web/'
process:
  fid: fid
  filename: filename
  source_full_path:
    -
      plugin: concat
      delimiter: /
      source:
        - constants/source_base_path
        - filepath
    -
      plugin: urlencode
  uri:
    plugin: file_copy
    source:
      - '@source_full_path'
      - uri
  filemime: filemime
  status: status
  created: timestamp
  changed: timestamp
  uid: uid
destination:
  plugin: entity:file
migration_dependencies: {}


And I am using this next snippet to link the migrated image files to the user picture field (This is used in my user migration file): 


  migrate_plus.migration.custom_user.yml


  user_picture: # Image field name in Drupal 8 site
    plugin: iterator
    source: user_picture # Image field name in Drupal 7 site
    process:
      target_id: fid
      alt: alt
      title: title
      width: width
      height: height

migration_dependencies:
  required:
    - drupal7_image


I can see that the files are being migrated and downloaded to my Drupal 8 installation but when I check the users I don't see any user pictures (fields are blank).

I really don't know what I am missing, I searched for examples on the Internet but I didn't find much.

Any help would be much appreciated. Thanks in advance. ;)
Thanks a lot @MrD I tested using the migration_lookup plugin and it works like a charm:

  user_picture:
    -
      plugin: default_value
      source: picture
      default_value: null
    -
      plugin: migration_lookup
      migration: drupal7_image