metaquery-image-plugin.js

( function ( document, metaQuery) {
  var imageElements = document.getElementsByTagName( 'img' ),

  updateElements = function ( matches, name ) {
    if ( !matches ) { return; }

    imageElements.forEach( setSrc );
  },

  setSrc = function ( element ) {
    var template = element.getAttribute( 'data-mq-src' );
    if ( template ) {
      element.src = template.replace( '[breakpoint]', name );
    }
  };

  metaQuery.onBreakpointChange( function ( matches, breakpointName ) {
    updateElements ( matches, breakpointName );
  });
}( document, metaQuery ));