I'm not sure if this is the preferred way to get a background keyed in but I added 2 sources, the Facetime camera and an Image. The camera has a Chroma Key (green) set and OBS now shows the image behind me with the green screen black.
If there is a better way, someone please let me know. However, now for the $64K question. I want to get the video stream out to Skype or Facetime. Does OBS provide any support to do this? For example, does it support Syphon (for output, as a server).