You can easily setup CDN in Drupal by following these instructions:
- Download the latest version of Drupal CDN.
- Unzip it to Drupal modules folder.
- Go to Site Configuration > CDN Integration.
- In Setting tab, do the following:
- Status: Enabled
- Mode: Basic
- Show statistics for the current page: Disabled
Check your sites source code that it is using CDN.
Picture links should be replaced with CDN URL addresses.