There are two factors behind the image limit:
One is, as mentioned, the dialup users and the amount of time it takes multiple images to load.
The other is that people should stop and think before they post images of every step in a build; it just is not necessary and there are not that many people interested in 43 images of each model a person builds. It clogs up the Forum, makes the threads long, and defeats the purpose of our site.
For those who feel compelled to document their builds in detail, there are plenty of image-hosting sites out there, and we encourage people to go the Fotki or Photobucket-type route and link to their albums on our site. That way, those who really want to see all that detail can do so, and the majority, who do not, can quickly see what's going on in the thread and view what they wish.
And without an image limit, this would be (and was, for a time) completely out of control. We're an information site, not an image-hosting site. You can get plenty done with three images. Yes, some people will then post six times with three images in each post, but as notlezah said, they're only hurting themselves.
Some people will push any rule up to and beyond its limit, no matter what it is. And many members have been quite cooperative with the three-image rule, and use links to their images.
This has gone full-circle and beyond now, so I'm going to lock this and leave it available for reference.