Images are always displayed as changed if thumbnails are activated

Problem/Motivation

This issue was found running the content moderation module. As described in the title, if the option to show a thumbnail is set for a field in the diff module settings, it is shown no matter what in split view.

Changes however are detected correctly, as was observed by removing the option to show thumbnails. Then, the field is removed if no changes were made and added with the picture's source if there were.

Steps to reproduce

1. Install the Content Moderation module
2. Install the Diff Module
3. Make sure to configure the image field of the standard "article" content type in the diff settings so that thumbnails are shown
4. Create a new article node with an image inside
5. Create another revision of the same node with some text changed
6. Navigate to the "revisions" view of the node
7. Compare the two created revisions
8. Select "Split view"
9. You should now see the image thumbnails of the revisions, despite not changing them

Proposed resolution

Thumbnails should only be shown if changes have been made to the image field in question.