Вы можете сделать следующее в PHP:
function base64_to_jpeg( $base64_string, $output_file ) {
$ifp = fopen( $output_file, "wb" );
fwrite( $ifp, base64_decode( $base64_string) );
fclose( $ifp );
return( $output_file );
}
$facebook->setFileUploadSupport(true);
$image = base64_to_jpeg( $your_base64_string, 'tmp.jpg' );
$args = array('message' => 'Some message');
$args['image'] = '@' . realpath( $image );
$data = $facebook->api('/your_user_id/photos', 'post', $args);
unlink($image);