On other vBulletin sites, I can do this:

{url=url-of-big-image}{img}url-of-small-image{/img}{/url}

Which will inline the small image in the post, and clicking on it will show the big image.

I just tried that here, and it seems to be very broken. Even with un-checking the "automatically parse links", the editor tries (and fails) to parse the links, and you end up with:

[img]http://www.atomacrossamerica.org/6F5S8418-s.jpg[/img]

It turns the {img} tag into a hotlink for the large image and doesn't inline the small image, instead leaving it as a plain link.

I ended up using "Insert Image" from the tool bar and adding some "click here for big version" text.

What gives?