If you have upgraded to WordPress 3.5 and also updated the theme, but you are still getting the following error displayed on your page:

Fatal error: Call to undefined method WP_Image_Editor::get_instance()

Then here is how to fix this error.

Find and open the following file:

PHP Code:
// replace this line
$editor WP_Image_Editor::get_instance($file);
// with
$editor wp_get_image_editor($file); 
We will be rolling out updates to fix this error shortly, but the above fix will help if you are need it urgently.