functions.php problem with uploading images

Hi,
I have my theme and after uploading functions.php file on my server i cant upload images as post thumbnail.

my functions.php is only this code:

<?php
add_theme_support( 'post-thumbnails' );
	set_post_thumbnail_size( 624, 9999 );
?>

also when i try to post something i got an error

Warning: Cannot modify header information – headers already sent by (output started at /wp/wp-content/themes/korki/functions.php:6) in /wp/wp-includes/pluggable.php on line 876