Hi Achim,
Ouch – you modified the core theme so much you can’t implement your changes in a child theme? Missing out on updates really sucks, as I’m always improving the theme, including the performance of it.
I’m not too sure about your question though – the full desktop image is still shown on tablets and even mobile, it’s just sized down to fit inside the container that it’s in. If it didn’t do this, it would reach outside of the container and look pretty bad.