Full page background images CSS

If you want to cover the background of your web page, I suggest you do 2 things:
Reduce the file size of the image using the technique shown at https://orbitingweb.com/blog/optimizing-jpeg-images/. You should make sure you are using the correct image format. Here is a guide https://www.diffen.com/difference/JPEG_vs_PNG.
Use the following CSS code in your stylesheet:
html {
  background: url(img/bg.jpg) no-repeat center center fixed;
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
  -ms-filter: "progid:DXImageTransform.Microsoft.AlphaImageLoader(src='img/bg.jpg', sizingMethod='scale')";
  filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(src='.img/bg.jpg', sizingMethod='scale');
}