Index: js/image_nodes.js
===================================================================
--- js/image_nodes.js	(revision 37)
+++ js/image_nodes.js	(working copy)
@@ -32,9 +32,9 @@
           if (settings.group_images) {
             rel = "lightbox[node_thumbnails]";
           }
-
+          
           // set the href attribute
-          var href = $(child).attr("src").replace(".thumbnail", "").replace(/(image\/view\/\d+)(\/\w*)/, "$1/_original");
+          var href = $(child).attr("src").replace(".thumbnail", settings.derivative).replace(/(image\/view\/\d+)(\/\w*)/, "$1/_original");
 
           // handle flickr images
           if ($(child).attr("class").match("flickr-photo-img")) {
Index: lightbox2.install
===================================================================
--- lightbox2.install	(revision 37)
+++ lightbox2.install	(working copy)
@@ -20,6 +20,7 @@
   variable_del('lightbox2_lite');
 
   variable_del('lightbox2_use_alt_layout');
+  variable_del('lightbox2_derivative');
   variable_del('lightbox2_force_show_nav');
   variable_del('lightbox2_image_count_str');
   variable_del('lightbox2_disable_zoom');
Index: lightbox2.module
===================================================================
--- lightbox2.module	(revision 37)
+++ lightbox2.module	(working copy)
@@ -151,6 +151,28 @@
     '#description' => t('Enabling this option alters the layout of the lightbox elements.  This doesn\'t apply when using Lightbox Lite.'),
     '#default_value' => variable_get('lightbox2_use_alt_layout', false),
   );
+  
+  
+  // Add DropDown for list of available derivatives.
+  if (module_exists('image')) {
+    $options = array('original' => t('original'));
+    $sizes = _image_get_sizes();
+    foreach ($sizes as $size_key=>$size) {
+      if ($size_key=='original' or $size_key=='_original')  {
+      	  $size_key='';
+	  } else {
+	  	  $size_key='.'.$size_key;
+	  }
+      $options[$size_key] = $size['label'];
+    }
+    $form['lightbox2_general_options']['lightbox2_derivative'] = array(
+      '#type' => 'select',
+      '#title' => t('Image derivative'),
+      '#options' => $options,
+      '#default_value' => variable_get('lightbox2_derivative', 'preview'),
+      '#description' => t('Select which image derivative will be loaded.'),
+    );
+  }
 
   // Add Checkbox for Force Navigation display
   $form['lightbox2_general_options']['lightbox2_force_show_nav'] = array(
@@ -302,6 +324,7 @@
 
   // load the javascript settings
   $js_settings = array(
+  	'derivative' => variable_get('lightbox2_derivative', false),
     'use_alt_layout' => variable_get('lightbox2_use_alt_layout', false),
     'disable_zoom' => variable_get('lightbox2_disable_zoom', false),
     'force_show_nav' => variable_get('lightbox2_force_show_nav', false),