reverted:
--- b/core/modules/migrate_drupal/config/install/migrate.migration.d6_user.yml
+++ a/core/modules/migrate_drupal/config/install/migrate.migration.d6_user.yml
@@ -23,13 +23,8 @@
     migration: d6_user_role
     source: roles
   user_picture:
+    plugin: d6_user_picture
+    source: uid
-    -
-      plugin: skip_process_on_empty
-      source: picture
-    -
-      plugin: migration
-      migration: d6_user_picture_file
-      source: uid
 destination:
   plugin: entity:user
   md5_passwords: true
reverted:
--- /dev/null
+++ a/core/modules/migrate_drupal/src/Plugin/migrate/process/d6/UserPicture.php
@@ -0,0 +1,63 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\migrate_drupal\Plugin\migrate\process\d6\UserPicture.
+ */
+
+namespace Drupal\migrate_drupal\Plugin\migrate\process\d6;
+
+use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\migrate\Entity\MigrationInterface;
+use Drupal\migrate\MigrateExecutableInterface;
+use Drupal\migrate\Plugin\MigrateProcessInterface;
+use Drupal\migrate\ProcessPluginBase;
+use Drupal\migrate\Row;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * The user picture process plugin.
+ *
+ * @MigrateProcessPlugin(
+ *   id = "d6_user_picture"
+ * )
+ */
+class UserPicture extends ProcessPluginBase implements ContainerFactoryPluginInterface {
+
+  /**
+   * The migration plugin.
+   *
+   * @var \Drupal\migrate\Plugin\MigrateProcessInterface
+   */
+  protected $migrationPlugin;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function __construct(array $configuration, $plugin_id, array $plugin_definition, MigrationInterface $migration, MigrateProcessInterface $migration_plugin) {
+    parent::__construct($configuration, $plugin_id, $plugin_definition);
+    $this->migration = $migration;
+    $this->migrationPlugin = $migration_plugin;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration = NULL) {
+    return new static(
+      $configuration,
+      $plugin_id,
+      $plugin_definition,
+      $migration,
+      $container->get('plugin.manager.migrate.process')->createInstance('migration', array('migration' => 'd6_user_picture_file'), $migration)
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
+    return $row->getSourceProperty('picture') ? $this->migrationPlugin->transform($value, $migrate_executable, $row, $destination_property) : NULL;
+  }
+
+}
only in patch2:
unchanged:
--- a/core/modules/user/src/Plugin/migrate/process/d6/UserPicture.php
+++ b/core/modules/user/src/Plugin/migrate/process/d6/UserPicture.php
@@ -57,7 +57,12 @@ public static function create(ContainerInterface $container, array $configuratio
    * {@inheritdoc}
    */
   public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
-    return $row->getSourceProperty('picture') ? $this->migrationPlugin->transform($value, $migrate_executable, $row, $destination_property) : NULL;
+    $fname = basename($row->getSourceProperty('picture'));
+    $fids = \Drupal::entityQuery('file')
+      ->condition('uid', $value, '=')
+      ->condition('filename', $fname, 'LIKE')
+      ->execute();
+    return (count($fids) == 1) ? reset($fids) : NULL;
   }
 
 }
only in patch2:
unchanged:
--- a/core/modules/user/src/Tests/Migrate/d6/MigrateUserTest.php
+++ b/core/modules/user/src/Tests/Migrate/d6/MigrateUserTest.php
@@ -60,7 +60,7 @@ protected function setUp() {
     ))->save();
 
     $file = entity_create('file', array(
-      'fid' => 2,
+      'fid' => 12,
       'uid' => 2,
       'filename' => 'image-test.jpg',
       'uri' => "public://image-test.jpg",
@@ -74,7 +74,7 @@ protected function setUp() {
     $file->save();
 
     $file = entity_create('file', array(
-      'fid' => 8,
+      'fid' => 18,
       'uid' => 8,
       'filename' => 'image-test.png',
       'uri' => "public://image-test.png",