Register image sizes - WordPress

register-img-sizes-wp.php

/* register additional image sizes */
function ak_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 ak_custom_image_setup () {
  add_theme_support( 'post-thumbnails' );
  add_image_size('slider-big', 787, 325, TRUE);
  add_image_size('slider-small', 587, 242, TRUE);
  add_image_size('header-big', 787, 200, TRUE);
  add_image_size('header-small', 500, 200, TRUE);
  add_image_size('gallery-small', 425);
  add_filter( 'image_size_names_choose', 'ak_insert_custom_image_sizes' );
}
add_action( 'after_setup_theme', 'ak_custom_image_setup' );