From MozillaZine Knowledge Base
Thunderbird doesn't provide a way to add more emoticons to the "insert a smiley face" list box used when composing a HTML message. You need to embed a .GIF file for the emoticon/smiley face in the message. The SmilieInserter Plus add-on makes it easier to do that. You used to also be able to do that using the MailTagger add-on but it is no longer maintained, and does not work with recent versions of Thunderbird if you disable the version check by installing the Disable Add-on Compatibility Check add-on beforehand .
When you add a emoticon using the "insert a smiley face" list box its actually stored as smiley face (text) characters in the message. Thunderbird displays any smiley face characters as a graphics image while other email clients display the smiley face characters as is. This is a built-in feature enabled by the mail.display_glyph setting and why what you see and other users see may differ. However, if you embed a .GIF file using one of the add-ons everybody sees a graphics image.
PostSmile is a standalone Windows utility that lets you add a emoticon to a message by dragging and dropping it into the message body. There are both free and premium versions, the main difference is the number of emoticons bundled with it.