I'm not sure i found the file in 4 image and changed the 0 - 1
// If you use GD higher 2.0.1 and PHP higher 4.0.6 set this to 1.
// Your thumbnails will be created with better quality
define('CONVERT_IS_GD2', 1);
But this hasn't changed the quality of the thumbnail.
It also gives pictures a bad quality if i re-size them.
GD library never give you better quality for resized photos, its god for thumbnails but not for resizing photos from big one,
you need change too much parameters in resizing process and for that you need some special Photographic software like PhotoShop, or
use ImageMagick on your server.