Don't use the Poser "background" for a background image if you find yourself wanting to scale or move the background.

Instead, go to the Primitive props and load a one-sided square. Examine the dimensions of your image, and copy those to the x scale and y scale of the square. For example, if your image is 1280 by 1024, put x scale = 1280% and y scale=1024%. This will give the prop the correct aspect ratio, and also make it nice and big.

Go into the material room for the square. Load your image into an Image_Map and plug that into Alternate_Diffuse of the Poser Surface root node. Turn off all other material effects, i.e. set Specular_Value = 0 and Diffuse_Value = 0.

We use this material arrangement so that the prop does not get lit or receive shadows. We don't want to light the backround photo.

Go back to the Pose room. Your square is now ready to manipulate. You can position and rotate the square any way you like so that you see what you want to see behind your figures and props.

If you want to change the size of the square (now a rectangle of course!) use the overall Scale parameter. This will keep the correct aspect ratio at all times.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)