Just wanted to mention that, after messing around with creating BillboardCollections, Billboards, and TextureAtlas's and seeing an alternative way to do it, I really think it would be beneficial to others for you guys to provide an example in the Demos/SandCastle of creating a TextureAtlas using the TextureAtlasBuilder. Using the method shown in the Sandcastle of dynamically creating an image and then assigning the src aftward and using the onload event just.. seems like a weird way to do it. Using the TextureAtlasBuilder to create a TextureAtlas from an icon is super simple and seems like a much easier way to do this.
Also, I know the documentation is still a work in progress but the documentation for the TextureAtlasBuilder doesn't show that the constructor accepts a TextureAtlas as a parameter.