We'd need more information from you to identify what the problem is. I assume the preferred server is set up correctly with the right credentials, correct? Have you attempted pointing the template directly at the preferred server as a test to see if it works in the first place? Is the image file in the exact same location on the preferred server as it is on the core? What does the maptopreferredhandler log say under X:/ldprovision while the device is in WinPE?