Call Jetpack_PostImages to resize thumbnails via Photon

photon-images.php

<a href="<?php echo get_the_permalink(); ?>">
<?php
    //check if Jetpack is acctive
    if (class_exists('Jetpack_PostImages')) {
    //get the ID of the post thumbnail image
    $thumb_id = get_post_thumbnail_id();
        //get the URL of the image
        $img_url = wp_get_attachment_image_src($thumb_id, 'full');
        //get the image alt text
        $alt = get_post_meta($thumb_id, '_wp_attachment_image_alt', true);
        //if not alt text, use the title
        if (empty($alt)) {
            $alt = get_the_title();
        }
        //if there is a post thumbnail image URL
        if ( !empty( $img_url ) ) {
            //call Jetpack_PostImages to retreive a 350px x 200px image from Photon
            $img_resize = Jetpack_PostImages::fit_image_url ($img_url[0], 350, 200);
            //display the image
            echo '<img src="' . $img_resize . '" alt="' . $alt . '">';
        }
    } else {
        //fallback to WordPress post thumbnail if Jetpack not enabled
        echo get_the_post_thumbnail($post->ID,'medium'));
    } 
?>
</a>