Some extra thoughts on using a square for a background image.

"Hi Res" in the 'Square Hi-Res' refers to the polygon density, which as far as I know should have no effect on image maps used on it. However the UV map on the Poser 'one sided square' (post P6 or 7) is wrong (the image overlaps the square).

A background image is not affected by the scene lighting, but by default a Poser prop is. You can make the image independent of the lighting as follows. In the Advanced tab of Material Room, use these settings:

Defuse_Color = white
Defuse_Value = 0
Specular_Color = Black
Specular_Value = 0
Ambient_Color = white
Ambient_Value = 1

Create an Image_Map node and plug it into both the Ambient_Color and Defuse_Color. Set the Filtering to 'Crisp'. Load your graphic into the node. You should also turn off 'Casts shadows' in the prop's Properties tab.

Another thing you can do is to parent the square to the Main Camera (for example), so that the square will move with the camera, then you don't need to adjust it every time you move the camera.

Attached above is the text of a square prop that has been set up as above, and is smart-proped to the Main Camera. You will need to remove the ".TXT" part of the file extension and place it in a props folder, before you can use it in Poser.

To use; load it in poser, load an image into its Image_Map node. If necessary zTran it until it is behind all the elements of your scene. Adjust the scale of the prop until it just fills the view-port.