Hello KingsruleActually, the more I think about it, this doesn't make sense unless the Rennlist server is much different. They have been around a lot longer than WBF and I'll surmise their picture libraries are even more than WBF. Also, aren't images "served" only if they are scrolled to?
In any case if WBF wants to be leading edge in substance as well as looks infinite scrolling should not just be dismissed. Maybe not for today but it should be in the near future evolution plan.
Yes, it is sometimes called 'lazy load' and is based on Ajax script. The call to display a thousand posts in a window is not just about the attachments, it is also the load on the MySQL database. And if you have many user online at once, the resource piles up. It is not a good use of power IMO.
It is also about the ability to get to a section of a long thread. On head-fi-org I have read some threads with 250 pages. And to get back to the section where I left off the day before, I had to treat it like a book, go to page 133 for example. That would be impossible with an infinite scroll.