Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.4.14)
// Compass (v1.0.3)
// ----

// Create a function that inserts the string `@2x` right before the filename
// extension (you may assume the extension has a length of 3 characters).

@function retinize($filename) {
  @return str-insert($filename, '@2x', str-index($filename, '.')); // make magic here
  //@return str-insert( $filename, '@2x', -6)
}

.foo {
  background: url("some-image.png");

  @media retina { // this is pseudo-code
    background: url( retinize("some-image.png.webp.apng") );
  }
}

SassMeister-output.css

.foo {
  background: url("some-image.png");
}
@media retina {
  .foo {
    background: url("some-image@2x.png.webp.apng");
  }
}