get_intermediate_image_sizes()

function get_intermediate_image_sizes() {
	$_wp_additional_image_sizes = wp_get_additional_image_sizes();
	$image_sizes = array('thumbnail', 'medium', 'medium_large', 'large'); // Standard sizes
	if ( ! empty( $_wp_additional_image_sizes ) ) {
		$image_sizes = array_merge( $image_sizes, array_keys( $_wp_additional_image_sizes ) );
	}

	/**
	 * Filters the list of intermediate image sizes.
	 *
	 * @since WP-2.5.0
	 *
	 * @param array $image_sizes An array of intermediate image sizes. Defaults
	 *                           are 'thumbnail', 'medium', 'medium_large', 'large'.
	 */
	return apply_filters( 'intermediate_image_sizes', $image_sizes );
}