In Confluence 3, all images displayed in original size with
!image.png|border=0!
didn't have the fancybox / zoom feature. That's ok, because content author didn't want this!

If author wants a thumbnail / provides zoom feature, he had to add
!image.png|thumbnail,border=0!

As of the new RTE in Confluence 4, Images in Full-Size are

• automatically resized to page-width if they are wider than browser size
• always have a click action for fancybox / zoom feature

That's nonsense and disturbing because user will not see "more" than without zoom!