Add conversion/tracking pixel to section for specific post

You can use the wordpress Conditional Tags

You can add something like this to your functions.php file:

is_single( 'your-post-slug' )
    add_action( 'wp_head', 'your_function_with_pixel' )

There are a number of different conditional tags you can use. And your-post-slug can be substituted with the post id as well.