retina markers.js

// http://gmap3.net/forum/viewtopic.php?pid=3280#p3280

// the image is 50x78px and and it scaled down to 25x39px.
var markerIcon = new google.maps.MarkerImage('map-icon.png', null, null, null, new GMaps.Size(25, 39));
var marker = new google.maps.Marker({
  icon: markerIcon, 
  
  
   (function($) {
        get_device_pixel_ratio = function() {
            if (window.devicePixelRatio === undefined) return 1; // No pixel ratio available. Assume 1:1.
            return window.devicePixelRatio;
        }
    })(jQuery);
    jQuery(document).ready(function($) {
        
        // CHECK PIXEL RATIO
        var pixel_ratio = get_device_pixel_ratio();
        var marker_size = pixel_ratio > 1.5 ? '@2x' : '';
    });



Then for the Map JS, do this:
options:{
      icon: new google.maps.MarkerImage('<?php bloginfo("template_url"); ?>/images/map-marker' + marker_size + '.png', null, null, null, new google.maps.Size(46,38))
                            }
// Using this example, you need to have 2 different marker files:
// map-marker.png
// map-marker@2x.png

// You can see my website for reference where I use markers:
// adventures.alexrakic.com/map/