Sorry - missed this post earlier.
For background images you need a
Ground Overlay, which is just an image that has been calibrated in Google Earth. There is a tutorial on how to do this here:
https://www.google.com/earth/outreach/learn/creating-photos-image-overlays-in-google-earth/. You then need to save your overlay to disk, which will result in a KML file and an associated image file (whichever image file you used to create to create the overlay with). Ideally, the KML and image file will be in the same disk location. To import the image into RM, select the KML file (
not the image file) and the image will be imported and located in the geographical position that you specified in Google Earth.