Wouldn't it be easier to add a .htaccess file inside your images directory that will allow only your site to pull stuff from it. So lets say you have that file in there and inside images directory you have happy.jpg. if you wish you could specify a different link when they request that. so when they add
www.domain.com/images/happy.jpg it would send them
www.domain.com/images/stolen.jpg and on that picture you can add anything such as "Stolen from DomainForums". I think this might be easier then changing the html of every smilie.