I'm definitely fine with intermediate steps as long as we're reasonably confident we're not designing ourselves into a corner.  I guess the main question is: how likely is it that someone is going to want to put any material on a screen overlay other than an image or video material?  It's hard for me to think of a use-case, but maybe I just lack imagination.

If I'm right, then maybe the image or video should be specified right in the screen overlay CZML, rather than indirectly through a material.  So the property might be `image` for now, and later we can add a `markdown` property.  Of course, we can add a `markdown` property later even if we have a `material` property now, so in that sense the decision isn't that critical.  Still, we should avoid confusing abstractions wherever possible.  An `image` property will be immediately obvious to people.  A `material` property where one of the types of material an image is much less intuitive, I think.