There are better methods, but mine (second-hand) is: get a square image you like and render it, Display>Paste onto background, slide the camera forward so that your actual elements are behind it and all you can see is your rendered background (Poser will switch back to unrendered view), save your library entry. When you save anything to a library Poser uses the current working view for the thumbnail, so this way you get your image. Display>Clear background image then leaves you free for the next one.