Code goes to your active theme’s functions.php file:

1. Use this custom function:

function save_base64image_to_post_attachment( $base64_img, $title ) {
    // Make sure that this file is included, as wp_generate_attachment_metadata() depends on it.
    require_once( ABSPATH . 'wp-admin/includes/image.php' );

    // Upload dir.
    $upload_dir  = wp_upload_dir();
    $upload_path = str_replace( '/', DIRECTORY_SEPARATOR, $upload_dir['path'] ) . DIRECTORY_SEPARATOR;

    $img             = str_replace( 'data:image/jpeg;base64,', '', $base64_img );
    $img             = str_replace( ' ', '+', $img );
    $decoded         = base64_decode( $img );
    $filename        = $title . '.jpeg';
    $file_type       = 'image/jpeg';
    $hashed_filename = md5( $filename . microtime() ) . '_' . $filename;

    // Save the image in the uploads directory.
    $upload_file = file_put_contents( $upload_path . $hashed_filename, $decoded );

    $attachment = array(
        'post_mime_type' => $file_type,
        'post_title'     => preg_replace( '/\.[^.]+$/', '', basename( $hashed_filename ) ),
        'post_content'   => '',
        'post_status'    => 'inherit',
        'guid'           => $upload_dir['url'] . '/' . basename( $hashed_filename )
    );

    $attach_id = wp_insert_attachment( $attachment, $upload_dir['path'] . '/' . $hashed_filename );
    // Generate the metadata for the attachment, and update the database record.
    $attach_data = wp_generate_attachment_metadata( $attach_id, $upload_dir['path'] . '/' . $hashed_filename );
    wp_update_attachment_metadata( $attach_id, $attach_data );

    return $attach_id;
}

2. This will be the hook that will save the attachment ids to the wpcargo shipment created:

function wpcargo_api_after_add_shipment_save_images_cb($shipmentID, $request) {
    $custom_field_meta_key = 'custom_shipment_images'; // change this one with your custom field file meta key
    $pre_shipment_images   = $request->get_param( $custom_field_meta_key );
    $attachment_ids    = array();
    if($pre_shipment_images && is_array($pre_shipment_images)) {
        $counter = 1;
        foreach($pre_shipment_images as $base64) {
            $title = 'image-' . $counter . '-' . time();
            $att_id = save_base64image_to_post_attachment($base64, $title);
            if($att_id && $att_id > 0) {
                if(!in_array($att_id, $attachment_ids)) {
                    $attachment_ids[] = $att_id;
                }
            }
            $counter++;
        }
    }

    if($attachment_ids && is_array($attachment_ids)) {
        $attachment_ids_imploded = implode(',', $attachment_ids);
        update_post_meta($shipmentID, $custom_field_meta_key, $attachment_ids_imploded);
    }
}
add_action('wpcargo_api_after_add_shipment', 'wpcargo_api_after_add_shipment_save_images_cb', 99, 2);