How are you uploading the images for the users? If you are using the image upload widget did you set a thumbnail size? Does the user uploading have access to the thumbnail to write? Do users reading the thumbnail have read access to it?

If the thumbnails are all empty but you have image content, then you may be able to recreate them using the DuplicateImageDocument action the Community Commons module in the Marketplace. You would have to iterate through all the Image documents you have, duplicate them, then use that duplicate instead instead of the original. Remember to set a sensible thumbnail size when calling this action so thumbnails are generated as part of the duplication process.

Good luck!