Quote - Back again. Poser2012...Render window is 873 x 844.  Background image is 2600 x 3800... Per attached you can see what happens.  It does not fill up the whole screen.

Daz3 np... Have no clue unless you  cannot use purchased backgrounds in Poser

Your camera focal length... 25? so, wide-angle?

You're best off getting some panorama obj or other plane (cloth_plane in Poser primitives in your props library) to display that image against - you would do this in the material room with a ImageMap node. Then, you can adjust the size (scale) of the plane and also of your camera focal length and have better control over the background image.

Monterey/Mint21.x/Win10 - Blender3.x - PP11.3(cm) - Musescore3.6.2

Wir sind gewohnt, daß die Menschen verhöhnen was sie nicht verstehen
[it is clear that humans have contempt for that which they do not understand] 

Metaphor of Chooks