Ok, perhaps this will help you understand what is wrong.
Image:
http://motomechtech.net/mmtgallery/details.php?image_id=107the original image is in
data/media/8/Harley-Davidson_Dyna_Glide_2005_12_1024x768.jpgbut there is no thumbnail for it, no such file:
data/thumbnail/8/Harley-Davidson_Dyna_Glide_2005_12_1024x768.jpgthe thumbnail you have is:
data/thumbnails/8/tn_Harley-Davidson_Dyna_Glide_2005_12_1024x768.jpgThumbnail filename must be the same as the image filename.
Once you got the thumbnail filename matching image filename, you can run the auto-thumbnailer mod I've mentioned above and it should find the missing thumbnails and update the database.
Or, if you don't mind a long way, edit the image and upload thumbnail from the edit page, 4images will do all necessary changes to the filename itself.