First, set your Document Window to Cartoon+Line mode (hotkey CTRL-7).

Render your images as animations. This way, you can render in whatever mode your display is currently set to, and you can render to any image size, regardless of your current screen resolution.

In the Animation menu, select Animation Setup. Input whatever size you want your image to be in Frame. Set Frame Count to 1, since you only want to render a single frame, and not an entire video. Click OK.

In the Animation menu, select Make Movie. As Sequence Type, choose Image Files, so you'll render an image rather than a video. Leave Resolution at Full. Set Quality to Current Display Settings (if your display is set to Cartoon+Line mode, this will be the mode it renders to). Turn on antialiasing for smoother lines. Click OK.

You'll be asked where to save the rendered image, and for filetype. Put it whereever you wish. Ideally, you should save to BMP or some other lossless format, rather than JPEG, since you might want to do postwork on the image.

Let Poser render and save your image. Cartoon+Line mode doesn't use textures, bump maps, or transparencies; what you see are the objects' material colors, plus outlines. If you want the image to be entirely in shades of grey, you should either change all of the model colors, or take the finished image into a paint program and convert it to greyscale.