[WordPress] Code from our blog post Adding Custom Image Sizes to WordPress 3.5 Media Manager - http://www.limecanvas.com/adding-custom-image-sizes-to-wordpress-3-5-media-manager/

code-snippet-1.php

// Add new image sizes
function lc_insert_custom_image_sizes( $image_sizes ) {
  // get the custom image sizes
  global $_wp_additional_image_sizes;
  // if there are none, just return the built-in sizes
  if ( empty( $_wp_additional_image_sizes ) )
    return $image_sizes;

  // add all the custom sizes to the built-in sizes
  foreach ( $_wp_additional_image_sizes as $id => $data ) {
    // take the size ID (e.g., 'my-name'), replace hyphens with spaces,
    // and capitalise the first letter of each word
    if ( !isset($image_sizes[$id]) )
      $image_sizes[$id] = ucfirst( str_replace( '-', ' ', $id ) );
    }

  return $image_sizes;
}

function lc_custom_image_setup () {
add_theme_support( 'post-thumbnails' );
add_image_size('Mini Square', 70, 70, TRUE);
add_image_size('Square', 100, 100, TRUE);
add_image_size('Featured Tabs', 150, 225, TRUE);
add_image_size('Featured', 640, 220, TRUE);
add_filter( 'image_size_names_choose', 'lc_insert_custom_image_sizes' );
}
add_action( 'after_setup_theme', 'lc_custom_image_setup' );