Executing `createimagefrompng()` from save_post hook (or equivalent hook)

The use of header() function produce output in the middle of the request and crash it.

To modify uploaded image you should use wp_handle_upload filter instead of save_post action. Also, ideally, you should use WP_Image_Editor class, which will use the best image manipulation library available in the server (GD, Imagemagick) and triggers several filters and actions that can be used by another plugins:

add_filter( 'wp_handle_upload', 'cyb_handle_upload_callback' );
function cyb_handle_upload_callback( $data ) {

    // get instance of WP_Image_Editor class
    $image = wp_get_image_editor( $data['file'] );

    if( ! is_wp_error( $image ) ) {

        // Manipulate your image here
        $stamp = imagecreatefrompng( 'path/to/stamp.png' );
        $margin_right = 10;
        $margeing_bottom = 10;
        $sx = imagesx( $stamp );
        $sy = imagesy( $stamp );

        imagecopy(
            $data['file'],
            $stamp,
            imagesx( $data['file'] ) - $sx - $margin_right,
            imagesy( $data['file'] ) - $sy - $margin_bottom,
            0,
            0,
            imagesx( $stamp ),
            imagesy( $stamp )
        );

        // Save the modified image
        $image->save();

    }

    return $data;
}

If we follow this implementation extending WP_Editor_Image class:

add_filter( 'wp_handle_upload', 'cyb_handle_upload_callback' );
function cyb_handle_upload_callback( $data ) {

    $image = wp_get_image_editor( $data['file'] );

    if ( ! is_wp_error( $image ) && is_callable( [ $image, 'stamp_watermark' ] ) ) {

        $stamp = imagecreatefrompng( '/path/to/stamp.png' );
        $success = $editor->stamp_watermark( $stamp );

        if ( ! is_wp_error( $success ) ) {
            $image->save();
        }

    }

    return $data;
}