A loading screen is a model with an image attached to it.
You can create a video by using a model with animations.
Each phase of the animation would show a different image because it would load a different 'texture'.
The problem is that it's not feasible in multiplayer maps because you're going to hit the 8MB limit fast :/
edit
Actually, your question is vague :V
We don't know if you're talking about the map description on this site, or inside the actual map during the loading screen phase.