function ec_after_setup() {
remove_action( 'after_setup_theme', 'extra_add_image_sizes' );
add_action( 'after_setup_theme', 'ec_add_image_sizes' );
}
add_action( 'after_setup_theme', 'ec_after_setup', 1 );

function ec_add_image_sizes() {
$sizes = array(
'extra-image-huge' => array(
'width' => 1280,
'height' => 768,
'crop' => true,
),
'extra-image-single-post' => array(
'width' => 1280,
'height' => 640,
'crop' => true,
),
'extra-image-medium' => array(
'width' => 627,
'height' => 999,
'crop' => true,
),
'extra-image-small' => array(
'width' => 440,
'height' => 264,
'crop' => true,
),
'extra-image-square-medium' => array(
'width' => 440,
'height' => 440,
'crop' => true,
),
'extra-image-square-small' => array(
'width' => 150,
'height' => 150,
'crop' => true,
),
);

foreach ( $sizes as $name => $size_info ) {
add_image_size( $name, $size_info['width'], $size_info['height'], $size_info['crop'] );
}
}