I see. Those thumbs are for use in the file manager. They crop from the top and bottom of images to make them fit properly on the page. There is nothing wrong with the thumb, it just isn't meant for a forward facing page like you are using it. It is only supposed to be used by the file manager and may crop the top and bottom or left and right of an image to fit.
If you want to have a thumb and have more control of the way the thumb is created, then you can put the full sized original image on the page and use the webassist "Image Resizer" to resize it in place to create the thumb.
Did this help? Tips are appreciated...
