Images (of movie clips) used as background in the 3D viewport will not render.

To add the 3D scene on to a movie clip add an alpha over node in the compostitor.

[![enter image description here][1]][1]


For single images the setup would be like this:

[![enter image description here][2]][2]

Make sure that you enable Film Transparent:

[![enter image description here][3]][3]


  [1]: https://i.sstatic.net/oEUTc.png
  [2]: https://i.sstatic.net/egSQs.png
  [3]: https://i.sstatic.net/MOnlM.png