That's probably because the thumbcodes are mostly aimed to show "normal" sized thumbnails in comments and not emoticons. It is worse when you add the Emoticon from the deviantArt tab in add media: see the space between them?
<*da:emoticon id="396698929" profile="deviation"> The number in this code is the same as the code in :*thumb396698929: . And no you can't use the emoticon id code for every emoticon, the emoticon has to be smaller than a certain size or you will see :bademoticon: And the code only works in the Emoticon gallery, but it can be used to display chat friendly emoticons that are in the Visual purpose Gallery (since those aren't shown in the Emoticon tab in add media) like:
I am a Premium member, but I'm on an XP machine with IE8!!! and although dA says it supports IE8 there have been several problems since the new comments upgrade. So the thumbnail codes don't work in comments for me even if I'm Premier, and also the thumbnails don't work on my profile page in IE either.