Frontsize - Example for backgroundRetinaResponsive mixin

app.less

...

// frontsize app.less file inside your theme folder
// you can override your @root-path like in background-size-responsive.less 

@use-root-path : true;
@root-path     : "/path/to/local/images/";

...

background-size-responsive.less

.my-super-background {

    .size(100%, 600px);

    @backgroundSize : cover;
    @baseUrl        : "http://placehold.it/";

    .backgroundRetinaResponsive(
        "1920x600&text=standard-background-on-full-hd",
        "3840x1200&text=retina-background-on-full-hd",
        "1280x600&text=standard-background-on-hd-ready",
        "2560x1200&text=retina-background-on-hd-ready",
        "1024x600&text=standard-background-on-tablet-landscape",
        "2048x1200&text=retina-background-on-tablet-landscape",
        "768x600&text=standard-background-on-tablet-portrait",
        "1536x1200&text=retina-background-on-tablet-portrait",
        "480x600&text=standard-background-on-smartphone-landscape",
        "960x1200&text=retina-background-on-smartphone-landscape",
        "320x600&text=standard-background-on-smartphone-portrait",
        "640x1200&text=retina-background-on-smartphone-portrait",
        @backgroundSize,
        @baseUrl
    );
}

example-background-retina-responsive.html

<div class="my-super-background"></div>

Frontsize Retina backgrounds wiki