We should use UserID object to identify users instead of username.

chrome/browser/chromeos/login/users/avatar/user_image_sync_observer.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/chromeos/login/users/avatar/user_image_sync_observer.h"
 
 #include "base/bind.h"
 #include "base/prefs/pref_change_registrar.h"
 #include "base/prefs/scoped_user_pref_update.h"
 #include "chrome/browser/chrome_notification_types.h"
@@ skipping 152 matching lines @@
   VLOG(1) << "Saved avatar index " << local_index << " to sync.";
 }
 
 void UserImageSyncObserver::UpdateLocalImageFromSynced() {
   int synced_index;
   GetSyncedImageIndex(&synced_index);
   int local_index = user_->image_index();
   if ((synced_index == local_index) || !IsIndexSupported(synced_index))
     return;
   UserImageManager* image_manager =
-      ChromeUserManager::Get()->GetUserImageManager(user_->email());
+      ChromeUserManager::Get()->GetUserImageManager(user_->GetUserID());
   if (synced_index == user_manager::User::USER_IMAGE_PROFILE) {
     image_manager->SaveUserImageFromProfileImage();
   } else {
     image_manager->SaveUserDefaultImageIndex(synced_index);
   }
   VLOG(1) << "Loaded avatar index " << synced_index << " from sync.";
 }
 
 bool UserImageSyncObserver::GetSyncedImageIndex(int* index) {
   *index = user_manager::User::USER_IMAGE_INVALID;
   const base::DictionaryValue* dict = prefs_->GetDictionary(kUserImageInfo);
   return dict && dict->GetInteger(kImageIndex, index);
 }
 
 bool UserImageSyncObserver::CanUpdateLocalImageNow() {
   if (WizardController* wizard_controller =
           WizardController::default_controller()) {
     UserImageScreen* screen = UserImageScreen::Get(wizard_controller);
     if (wizard_controller->current_screen() == screen) {
       if (screen->user_selected_image())
         return false;
     }
   }
   return true;
 }
 
 }  // namespace chromeos