slider-bg.jpg truncated when resized

Oh I see, since you don’t have any content inside of your widget it shows up like that on smaller devices. By default there is a top and bottom padding of 200px each. When resized or viewed in a mobile browser it turns to 50px since its viewed on a smaller screen. Since you’re just using the background image with no text in the middle, I would remove the code that overrides the main code so it stays at 200px top and bottom. You can do this by removing, commenting out, or editing/adjusting the padding on line 3647 in your css –

.slider .wrap {
    padding: 50px 0 30px;
}