I remember a few users requested support for custom smilies. With the new Auto Replace feature you have them.
First off all, you need to disable built-in smilies. Click Tools > Preferences… > Editor and clear “Replace smilies with images” check box.
Secondly, upload your smilies to your host.
Finally, click Tools > Auto Replace…, and assign words and HTML codes for your smilies. For example:
Word: :-)
Replace with: <img src="http://example.com/smile1.gif" alt=":)" height="15" width="15" />
Done! Now enjoy your custom smilies: 
See how Auto Replace is poweful?