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. However, the SmilieInserter Plus and MailTagger extensions for Thunderbird make it easier to do that. For SeaMonkey a modified version of MailTagger is available.
You may need to install the Disable Add-on Compatibility Check add-on beforehand if you want to install MailTagger since the last version targeted Thunderbird 3.0.
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.