You can easily setup CDN in Drupal by following these instructions:

  1. Download the latest version of Drupal CDN.
  2. Unzip it to Drupal modules folder.
  3. Go to Site Configuration > CDN Integration.
  4. In Setting tab, do the following:
    • Status: Enabled
    • Mode: Basic
    • Show statistics for the current page: Disabled
  5. Save
  6. In tab Basic Mode, enter CDN URL
  7. Save

Check your sites source code that it is using CDN.
Picture links should be replaced with CDN URL addresses.