In some cases the automatically-generated thumbnails created by Asset Bank for certain file types are not satisfactory. For example, if the source file contains transparency on a white background then the resulting JPEG thumbnail might look all white.
If you want to be able to upload a representative image (e.g. a JPEG) to be used as the thumbnail, for example that looks exactly how you want the thumbnail to look, then you can configure Asset Bank to show a 'Thumbnail' field on the 'Edit Asset' page. This works the same way as it does for non-image file formats as described in How to use a representative image as thumbnail for asset that is showing a generic icon
The method to get around this involves changing a record in the database that explicitly allows 'thumbnail files' to be uploaded for the given file extension.
The statement needed is:
UPDATE FileFormat SET CanUploadThumbnail = 1 WHERE FileExtension = 'jpg';
Replace jpg with the file extension of your choice. Once you have uploaded an Asset you can edit it to replace the thumbnail.