Watermark without plugins

Hello, I make adding watermark when image is loading into medialibrary. For that I use “add_attachment” hook:
add_action('add_attachment', 'applyWatermark');
My trouble is thumbnails has watermark too. I want that they didn’t have it.
I suspect that need find other hook who will executed after create thumbnail function but I can’t find it. Or exist other way perhaps?

Handler function “applyWatermark” if is needed:

function applyWatermark($attachId)
{
  if (wp_attachment_is_image($attachId)) {

    $attachPath = get_attached_file($attachId);
    $templatePath = get_bloginfo('template_directory');

    $im = imagecreatefromjpeg($attachPath);
    $stamp = imageCreateFromPng($templatePath.'/watermarks/wm.png');

    $marge_right = 10;
    $marge_bottom = 10;
    $sx = imagesx($stamp);
    $sy = imagesy($stamp);
    imageAlphaBlending($stamp, false);
    imageSaveAlpha($stamp, true);

    imagecopy($im, $stamp, imagesx($im) - $sx - $marge_right, imagesy($im) - $sy - $marge_bottom, 0, 0, imagesx($stamp), imagesy($stamp));

    imagejpeg($im, $attachPath);
    imagedestroy($im);
  }

  return $attachId;
}