Hi Jörg, currently background images in MoI are intended to be a reference aid for use while modeling, they don't export to different formats other than .3dm for reloading back into MoI.

There are some problem areas with trying to do that, for example PDF does not have a way to set up a perspective transform so I don't think it is feasible for images in the 3D view to get exported directly into a PDF output.

- Michael