Hi Chuck, yes TN = thumbnail.
I've just uploaded these 2 images using Imagemagick, quality of the large images is very good, but the thumbnails looks rather poor:
Digital-Fotofusion Gallery - MG_9609-1200x800 Digital-Fotofusion Gallery - MG_3007a-1024x768
The other 2 images in the 'Phil Preston' gallery were uploaded with GD, again the main image is good, and while the thumbnail is not great, its reasonable quality.
All were uploaded with quality setting of 100%