I have this problem in P6 too, never had it in 5. I have IC3 from RDNA and the backdrop on that can be scaled to any size so I use that in my scene and put my background image on it as a texture, works great. You can do the same with the DAZ Cyclorama! thefixer, poser coord.

Injustice will be avenged.
Cofiwch Dryweryn.