After you render, save the image as a .png file. When you open it in your graphic program, it will not have a background. You can also save it as a .psd file.

"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