Could be that your resolution is being reduced when Poser makes the thumbnail/rsr, maybe? 1. Render it as a 350x350 pixel image, and create your preview using a 350x350 render preview window.

or try

2. Render it as a larger image, import the square prop and use the render as a material on the prop. Arrange the prop so it fills the viewing window, and save under another name as before.

Those work for me. What file format are you saving the rendered image in?