In my preferences, I disabled "Show images in posts".

JPG images are replaced with links as expected. However, PNG images are still appearing.

EDIT: Now noticed some JPGs are still being shown too, for example the images in this post:
http://originaltrilogy.com/forum/topic.cfm/The-X-Project-Laserdisc-on-Steroids/post/334304/#post334304

--

A related suggestion for future enhancement - it would be nice if the option could be tailored to be more flexible, e.g. "Replace images with links if wider than 600px/larger than 100KB, etc."

That way, users can still see reasonably sized images, but when a post contains dozens of huge scans (because the ignorant poster has ignored requests to use thumbnail links instead) the large images are replaced with text links.

The ignore feature isn't much use in this case, as even though the posts themselves are not displayed, the images are still loaded as the page is rendered.