Slider unable to create settings for breakpoints

When I add a slider to a page I am unable to alter image size for a specific break point.

For instance I add in slider and then add the full size slider image in.

I then go to responsive mode and go to mobile. If I now re-size the image to be viewable on the mobile phone, it re-sizes it for all breakpoints. So when I go back to desktop the image is now small as its same size thats been set on mobile.

How do I set up the image sizes for the break points? Most sliders will scale the image but this one cuts off all the surrounding image and looks horrible on mobile.

Is this how you intend it or is there a bug?