Rated 5 out of 5 stars
Works even in TB3 if you disable the compatibility check.
You can combine this addon with the downloadable animated skype emoticons (https://developer.skype.com/Download/SkypeEmoticons) :-)
The link given by Ralphbusdriver provides a newer version (http://supersilly.com/~greg/storage/smilie_inserter-0.7.0-tb.xpi)
When you compase a message, you'll see a new smilie-button. Just click it, click "change" to set the image directory and you're all set