Are post thumbnails not working on WP 3.5?

The theme is not showing post thumbnails at all. Not just for old posts, but for new posts as well. I'm on Wordpress 3.5.

I checked with the "Theme Check" plugin for any compatibility issues and I think this particular error could be the cause of this issue:

[error]
RECOMMENDED: image_resize found in the file media.php. Deprecated since version 3.5. Use wp_get_image_editor() instead.
Line 19: * @uses image_resize_dimensions()
Line 20: * @uses image_resize()
Line 51: $dims = image_resize_dimensions($orig_w, $orig_h, $width, $height, $crop);
Line 92: $resized_img_path = image_resize( $img_path, $width, $height, $crop ); // Fallback foo
[/error]

Please reply asap. I need to set the theme up real quickly.

Thanks,
Shivam