title: fit background image to browser width and expand height if necessary
body: <p>i have portrait image (width: 869px; height: 2853px) which i wanna use as the background image for my website. the image should be responsive and always fill the entire width of the browser window. the image should keep its proportions and should never be cropped. therefor the height needs to adjust to the given width. since the image height is always bigger than the viewport height, you should be able to scroll to the bottom of the image, which should be the bottom of the website as well.</p>
  
  <p>i'd really appreciate if someone would tell me how to do this.</p>