Create An Alpha Channel/Transparency

[Gerhard Huber]

I want, that using the rubber makes the image transparent. How do I do that?
The rubber usually uses the background color. But if an image has an alpha
channel/transparency, using the rubber makes an image transparent. An alpha channel/transparency is created by the function Layer Properties. In this dialog there is a switch named Use Transparency which allows to both create and remove an alpha channel/transparency.