It used to be that if I clicked on a picture in a forum post, it would pop up over top of the discussion page. Now it's different, it links to a different page with just the picture. It's much slower now, waiting for the picture page to load and then having to click "back" and wait for the discussion page to load again.

Why is this? Is there any way to change it back to the old behavior?

Click on Preferences at the top of the page. In the Display Prefs section, find "When clicking thumbnails, show large image on a new page." Make your choice, then click Submit at the bottom of the page.