I'm wondering if wouldn't it be better if URLs to images would be outputted as normal links. I mean, not as product_thumb?image.jpg&w=80&h=80 but already to resized image like images/image.jpg.thumb_80x80.jpg.
Thumbnails in this case would need to be generated a little bit sooner (when page is requested instead of when image itself is requested) but it has one advantage that I can think of - we would not need to mess with headers, browser and server would take care of it.