WordPress snippet for dealing with image sizes

sjf_deh_define_image_sizes.php

<?php

/**
 * Standard practice among WordPress themes is to declare the width of the main content column as a global.
 *
 * @var integer
 */
$content_width = 800;

/**
 * A function to grab the $content_width and register several image sizes for our theme
 *
 * Caveats:
 *  * I'm basically assuming that everyone has a retina monitor, which is objectionable.
 *  * I'm assuming that desktop full-width is around 2000px, which is pretty random.
 *  * I'm assuming that the mobile world starts around 960px, which is highly questionalble.
 */
function sjf_deh_define_image_sizes() {
	
	// Grab the $content_width global.
	global $content_width;
	add_image_size( 'sjf_deh_content_width', $content_width, 9999 );

	// For retinas, the content width is doubled to arrive at full-width.
	$retina_full = $content_width * 2;
	add_image_size( 'sjf_deh_masonry_retina_full', $retina_full, 9999 );
	
	// For retinas, the content width is used in full for half-width images.
	$retina_half = $content_width;
	add_image_size( 'sjf_deh_masonry_retina_half', $retina_half, 9999 );
	
	// For retinas, the content width is halved for quarter-width images.
	$retina_quarter = $content_width / 2;
	add_image_size( 'sjf_deh_masonry_retina_quarter', $retina_quarter, 9999 );
	
	// For full-page bg's lets say 2000px is a good width.  Pretty arbitrary.
	$sjf_bg_size = 2000;
	add_image_size( 'sjf_deh_bg', $sjf_bg_size, 9999 );
	
	// For full-page bg's for mobile folks, let's say 960px is a good width -- even more arbitrary.
	$sjf_mobile_bg_size = 960;
	add_image_size( 'sjf_deh_mobile_bg', $sjf_mobile_bg_size, 9999 );

}
add_action( 'init', 'sjf_deh_define_image_sizes' );