Frontsize - How to use backgroundRetina mixin

app.less

...

// case for first-background
@use-root-path : false;

...

// case for second-background
@use-root-path : true;
@root-path     : "http://placehold.it/";

...

// case for third-background, .third-background shows how to override the app.less @root-path
@use-root-path : true;
@root-path     : "http://my-website.com/";

...

retina-background.html

<div>First example</div>
<div class="first-background bottom-margin"></div>

<div>Second example</div>
<div class="second-background bottom-margin"></div>

<div>Third example</div>
<div class="third-background"></div>

set-retina-background.less

.first-background {
    .size(650px, 300px);
    .backgroundRetina("http://placehold.it/650x300", "http://placehold.it/1300x600", cover);
}

.second-background {
    .size(100%, 300px);
    .backgroundRetina("650x300", "1300x600", cover);
}

.third-background {
    .size(100%, 300px);
    .backgroundRetina("650x300", "1300x600", cover, "http://another-website.com/");
}

Frontsize Retina backgrounds wiki