Images uploaded to the DL portlet normally have thumbnails generated. This, however, does not occur when uploading to a third-party repository mounted to the DL. However, all other formats that generates thumbnails will attempt to create a thumbnail regardless of the repository.
One of the major causes of this problem is the fact that the DLFileEntry / DLFileVersion tables have the columns:
Perhaps a new table designed specifically for Images in the DL should be created that contains these columns. This way, whether it is a DLFileEntry or a RepositoryEntry, it can still maintain the IDs for the thumbnails.