Button with transparent image

Hi,

I have a Button and assigned an Image in the Forms designer. It's a 32 bit Png image with an alpha channel. In the designer it looks as expected:

This means, the button's own color is seen in the transparent areas of the image.

But at runtime it looks different:

So, in the transparent areas of the image, the color of the button's container (blue) is seen.

Is there a setting that causes/prevents this at runtime? I don't want to write additional code.

Armin