I find that I get that option after a render if I have rendered in the pose window. It then asks me to save the .pz3 file first, and then it allows me to save the .png/.tif/.jpg image. I always render in a separate window (same size as my pose window 800 x 800) now. I always save the .pz3 before rendering anyway, this way I don't get asked to save it again after the render.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi