Hi. With "rotation png" do you mean a single image of a rotated planet? In that case you can just change the rotation slider and under export click "png".
If you want a animation of the rotation you can instead choose "spritesheet" and it will automatically generate a spritesheet with the animation for a single entire rotation. You then have to use a game engine or some other tool to splice the sheet and display it as an animation.
Hope that helps!