Warning: session_start(): Cannot send session cookie - headers already sent by (output started at /home/marleyla/public_html/sessione/addimg.php:2) in /home/marleyla/public_html/sessione/addimg.php on line 6

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/marleyla/public_html/sessione/addimg.php:2) in /home/marleyla/public_html/sessione/addimg.php on line 6

ho dei problemi con le sessioni, in locale funziona...

poi ho unaltro problema...
Warning: Cannot modify header information - headers already sent by (output started at /home/marleyla/public_html/sessione/addimg.php:2) in /home/marleyla/public_html/sessione/addimg.php on line 4

mi da questo errore con
header("Location: /index.php?id=".aggiungiimmagine($namef, $px, $py));

non capisco perchè... in locale funziona tutto