The thumbnails collection
The thumbnails collection in the Content Repository contains image thumbnails, which are dynamically created to make it easy to preview an image.
Once created, these image thumbnails are permanently stored so they do not have to be re-created each time you view an image. This collection is dynamically created after the first image thumbnail is created. Localization and published sub-collections are dynamically created when a user localizes or publishes images.
Thumbnails are created from the file designated as the default in an image object. If you designate the Hi-Res file as the default, be aware that your thumbnails might be very large and take a long time to render.