Recently I replaced my old, heavily modified Mile Stone 2 store with 2.3.1. I wanted some of the same contributions that I had installed 4-5 years ago. I installed 'On The Fly' Auto Thumbnailer and my thumbnails wouldn't display. I just saw the alt text as a link and the product title as a link. The thing that fixed my trouble was replacing the code in catalog/product_thumb.php with the code in this post:
http://www.oscommerce.com/forums/index.php?showtopic=285510&st=0&gopid=1188192&
I also found problems in my catalog\includes\functions\html_output.php because my html editor turned two & into & and | into |, which is a pipe, or vertical bar.
It was interesting to see my HTML editor screwing up, but fixing html_output.php with notepad still didn't display my thumbnails, the code in the link did.
Danny