A theme helper function for Photon (http://developer.wordpress.com/docs/photon/)

functions.php

<?php

function ch_photonize( $url = '', $params = array() ) {
	$server_pool = rand( 0,3 );
	return add_query_arg(
		$params,
		"https://i{$server_pool}.wp.com/" . str_replace( array( 'http://', 'https://' ), '', $url )
	);
}

while-loop.php

<?php

// Sample usage: (in a theme, within the loop)
// BTW: I know there are better ways to go about it, but this is just for demonstration purposes.

while ( have_posts() ) : the_post(); ?>

	<div class="post-featured-image">
	<?php if (has_post_thumbnail( $post->ID ) ): ?>
		<?php $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'single-post-thumbnail' ); ?>
		<img src="<?php echo ch_photonize( current($image), array( 'crop' => '300,300' ) ); ?>" alt="">
	<?php endif; ?>
	</div>

<?php endwhile; ?>