1920px is the max width of the banner area and self-adjusts based on the browser or viewing device. The image height will be relative to the width as the browser window shrinks. This is how the responsiveness works in websites. So for example, if your image is 1920 x 425 pixels, and you shrink your browser window by 50%, then your image will self adjust to 960 x 212. You can force the image to stay as a static size, but then you will lose the responsiveness of the theme.