@Robbie - In my opinion, you're playing with fire by allowing that kind of stuff. It definitely means keeping an eye on every comment for moderation. Wouldn't want people to embed stupid videos or insulting images. However, I'm going to test the YouTube embed code in this comment form to see if it works. Check out this tutorial on how to create a plugin that enables the submission of images from the front-end of the site to the WordPress media library http://wp.tutsplus.com/tutorials/allow-users-to-submit-images-your-site/

Another link worth checking out has about 33 plugins dealing with enhancing WordPress comments http://www.1stwebdesigner.com/wordpress/33-wordpress-plugins-to-power-up-your-comment-section/