7 files declare their use of ImageItem in Drupal 8

EditorMediaDialog.php in core/modules/media/src/Form/EditorMediaDialog.php

media.install in core/modules/media/media.install

Install, uninstall and update hooks for Media module.

MediaEmbed.php in core/modules/media/src/Plugin/Filter/MediaEmbed.php

media_library.module in core/modules/media_library/media_library.module

Contains hook implementations for the media_library module.

QuickEditImageController.php in core/modules/image/src/Controller/QuickEditImageController.php

user.module in core/modules/user/user.module

Enables the user registration and login system.

XmlEntityNormalizationQuirksTrait.php in core/modules/rest/tests/src/Functional/EntityResource/XmlEntityNormalizationQuirksTrait.php