If it's a link, just add the URL. Sometimes the link is more effective than embedding the video, especially if it's not a YouTube/Vimeo video.
You can't directly upload videos or photos as an attachment to GTPlanet...not even I have access to that. But there is a photo gallery included with each membership; check you profile for details, and you can hotlink an image between tags.
Example
[img]http://cache.gtpla.net/assets/11/images/rss.png[/img]