When previewing a new post, emoticons will be replaced with the correct .gif file on the initial preview.
However, if you click preview again, the gif will be replaced with the emoticon's text. This appears to happen with all emoticons.
For example, if I type in [colon]tama-mad[colon], then hit "preview" from the main thread, the preview will show
When I click the preview button again (when the formatting options and clickable emoticons are shown), the picture is not shown.
This is the case for both IE and Chrome.
Also, I'm not sure if this was how it worked before, but certain emoticons such as "thumb up" do not insert a trailing colon into the post text when clicked.