Rename _this into $this for consistency with $images. Also, use $this instead of the this keyword where possible, for better minification.

jquery.imagesloaded.js

@@ -17,13 +17,13 @@
 
 (function($, undefined) {
   $.fn.imagesLoaded = function( callback ) {
-    var $images = this.find('img'),
+    var $this = this,
+        $images = $this.find('img'),
         len = $images.length,
-        _this = this,
         blank = 'data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///ywAAAAAAQABAAACAUwAOw==';
 
     function triggerCallback() {
-      callback.call( _this, $images );
+      callback.call( $this, $images );
     }
 
     function imgLoaded() {
@@ -48,6 +48,6 @@
       }
     });
 
-    return this;
+    return $this;
   };
 })(jQuery);