img-load-masonry.js

// jQuery
$( function() {

  $('.js-masonry.imgload').each( function( i, elem ) {
    var $elem = $(elem).imagesLoaded( function() {
      var attr = $elem.attr('data-masonry-options');
      var options = attr && JSON.parse( attr );
      $elem.masonry( options );
    });
  });

});

// vanilla JS
docReady( function() {

  var elems = document.querySelectorAll('.js-masonry-imgload');

  function imgLoadMasonry( elem ) {
    var attr = elem.getAttribute('data-masonry-options');
    var options = attr && JSON.parse( attr );
    // init masonry after images have been loaded
    imagesLoaded( elem, function() {
      new Masonry( elem, options );
    });
  }

  for ( var i=0, len = elems.length; i < len; i++ ) {
    var elem = elems[i];
    imgLoadMasonry( elem );
  }

});