Use the Image button in the ribbon to insert an image:

Insert image in the ribbon

You now want to put in the location for the inserted image. Make sure that the Network Service has at least Read permissions:

Specify image location

Finally, you want to make sure that the image(s) are set to 'Include with the message' in both categories shown below:
  • Images Referenced by URLs
  • Images Referenced by Filenames

Signature Manager Outlook Edition How To