You need your background picture to be the same shape (ratio) as the document window. For example, if your render window is 1000x1000, you need your picture to also be 1000x1000 for best results - or it could be 2000x2000 (and Poser will shrink it) or 500x500 (and Poser will enlarge it). But it must be the same shape. If your render window is 800x600 then your background picture needs to be 800x600 or 1024x768 or 640x480... Also if you are rendering to a new window, you must make sure that your render window is the same shape as your document window, otherwise you will not get an accurate preview. Make sure "Constrain aspect ratio" is selected in the render options.

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"you are terrifying
and strange and beautiful
something not everyone knows how to love." - Warsan Shire