There are two "backdrop" image parameters in Carrara: 1) in the Vertex modeling room, 2) in the Assembly room (this would be the Global backdrop). They are not the same thing and using one does not affect the other.

In the vertex room, the backdrop images are only project on the working box planes that you select. Their placement should "stick" so you don't have to keep reloading them. These images will not show up in the Assembly Room or in the final render.

The Assembly Room (globabl) backdrop image does appear if you select that option in the interactive renderer. It also will show up in the final render. However, the Assembly Room doesn't let you apply the image to the working box planes - it only shows in the production frame and will be forced to fit the dimensions of your production frame.

Clear as mud?  ;)