Tested the background setup on a back plane, this works but naturally the size will not be adapted like the background to the Image. So the Issue is only concerning the background in fiefly  of poser 13. Also tested all the previous Poser versions if they are affected with this issue in case that a Win update caused a issue but all works like a charm in older poser versions.

Meaning all the background features in P13 are broken. Render dimension setup is not working in combination with a background, Importing a new background does not work, removing the background does not work, changing the Auto_ Fit feature in the material room does not work. The background feature is actually a essential feature for final renders if not filling up the whole scene with meshes without this feature working correctly Firefly is almost useless and broken.