funkiej It will work when you set image as background via CSS, not in this case - you use a module to show image.
For example, you have a section with specific class 'bgimage' - for instance, then you add style for that section:
.bgimage {
background-image: url("img_parallax.jpg");
background-attachment: fixed;
background-position: center;
background-repeat: no-repeat;
background-size: cover;
}
