This is a great concept but it gives me a couple questions.
Can you add permissions for a per-user basis?
When the user inputs a new keyword will all posts with that keyword use their smilie? or does the new smilie only show in the users posts? Is the new smilie parsed so it will not interfere with current keywords? (if not might I suggest AJAX to make this check on-the-fly?
Are there restrictions on type of images? Can you add an option to limit file types?
Are there instructions for adding the MySmilies dropdown for the Quick Reply box too?
edit...I would like to recommend adding a blank index.html to the zip for the mysmiliesvb directory for security