OK, try this. Make a new document and set the document window to be 600x600 pixels. Import a background picture which you have chosen and resized to 600x600 pixels. Go to Render Options. Select "Render to new window" and click the "Match Document Window" button (1) Then select "Constrain Aspect Ratios" (2) and set the render size you want. Just type in the first number and the second will be filled in. Render. Now tell me if you still have blank space above and below the background picture, or at either side. I think this will work for you. I had this problem a lot when I first started because I didn't understand that the new window render had to be the same proportions as the pose window - otherwise my preview would not be accurate.

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