Just a quick note – if You are experiencing missing thumbnails in image upload in WordPress, then probably Your PHP installation misses php-gd module.
It is module by default installed on FreeBSD, but on Debian based Linux You have to add it by hand (apt-get install php5-gd).
I had to move my blogs to new, temporary host, with Linux on board instead of FreeBSD. Then I have noticed that when I want to publish new post I can only insert in it uploaded image in original size – not very handy. It took 15 minutes of googling and finally dive into wp-includes/media.php to check how these thumbnails are generated.