Steps to reproduce:
1. Go to Documents and Media, and drag & drop a large amount of images into it (around 20 or more).
2. Without waiting for the whole process to finish, go to Blogs, and create a new blogs entry or edit an existing one.
3. Add an image to the content (the Item Selector popup should open).
4. Assert that some of the thumbnails are not displayed (expected).
5. Click on one of these thumbnails to open the preview.
Expected: The preview popup shows the full image if no preview exist.
Actual: The preview popup shows no content if the preview does not exist.