When composing a reply, a preview is shown (slightly grayed out) at the bottom of the discussion. You can quickly jump to the preview by clicking the eye icon in the edit toolbar text to the "Post reply" button.
When editing, the preview will be placed where the original post is. The eye icon will also jump to the correct place.
That preview should show any image you have in the post, either through bbcode, markdown, FoF Upload, AutoImage or MediaEmbed.
There is currently no preview when writing a new discussion.
Are you referring to something different?
