To my knowledge the ones in the library aren't transparent. They will have whatever background you assign to it.... if you save a prop or a light it will give you a .png of the image you are seeing on your screen. If you want a different .png thumbnail, you change the size of your render to 91 x 91 and then render over whatever background colour you want and then save it as the name of the old .png.

"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