I like this feature quite a bit. Perhaps it was in place on the old forums and I never noticed it.
Links to images included in posts (at least those hosted at imageshack, if not others too), when clicked, come up in a pop-up element on screen with a close button, rather than opening a new browser tab, window, or just sending the current tab to the image itself while losing the forum page. No need to click the back button after following an image link, just the close button.
It looks like this works in Firefox, Chrome, and MSIE, even if you are not logged in to the forum at the time.