4. "Paste" the URL into one of the text fields in the Personal Smilies section in your profile.
5. Click "set changes"
6. Scroll down and click "save account settings"
To use the personal smiley when creating or editing a post, click the "Insert Smiley" button (it has a smiley face on it; go figure!) and scroll down to the bottom to find the smilies that you've added.
Unfortunately, adding one of your personal smilies to a shoutbox post can be a tad inconvenient, but it's still very simple.
In a post to which you've added a smiley:
1. "edit" it
2. Select the "BBCode" view
3. Find the coded img tag (example shown below) and copy-and-paste it into the shoutbox. Bingo!