Using a header image in responsive design

Hello, I want to use a header image with our company logo in our header. The image should be located on the right side of the header. URL: http://blog.obo-bettermann.de/ The problem is: The header image does not scale in the responsive design. When I look at the page on my iPhone or similar mobile devices, the image look unproportionally large. Can you please help me with that? Kind regards Marvin Laryea