As i said before, you have to resize the image to build the tumbnail why you say that your tumbnail is autoresize???

PM me your blog link... anyways if you want to make a thumbnail from a larger image you have to resize it with css (wasting bandwith, because the size of the image remains the same you only rescale dimensions) with any php script (save banbwith, for example timthumb.php).

As example you can see this to be more clear:

Full size image: http://www.videoripx.com/wp-content/...-naughty-4.jpg

Thumbnail: http://www.videoripx.com/wp-content/...190&w=145&zc=1

Not sure of you are searching for something like that...