Homepage Slider size isn’t “Responsive”

I understand that the recommended image size for a homepage slider is 1920×700. In full-size desktop mode this is great. But this doesn’t change in responsive mode where an image of about 300×200 is ideal. Is there a recommended way to change the image or its sizing based on the form-factor currently in use? I already have images copied and resized for this purpose. But I need code or the theme to change the image by name or with CSS.

Thanks.