The Image Library in Content Management System is specific to that CMS site only. For images that you would like to have appear on more than one CMS site, they must be uploaded to each site individually or you can select them when adding an image through the Image icon in the editor. 

To upload an image to the specific CMS site
  1. Navigate to Content > Content Management System 
  2. Hover over the name of your CMS and click Manage Website
  3. Under Home > Home, click Add an Image underneath Files
  4. Click Choose File and select the image from your desktop
  5. Fill out the information for the Title. Repeat for up to 4 images. 
  6. Click Upload