Word Automation Service – Missing Images

Word Automation Services (WAS) is a SharePoint Server service application that enables unattended, server-side conversion of documents that are supported by the Microsoft Word client application. This section provides a general overview of Word Automation Services and reviews common business scenarios that the service was designed to support.
Whenever you are using WAS to save documents in various formats like .docx, .docm etc. and if the conversion output has missing images with the following message
The image part with relationship id was not found in the file
This could be because the Word Automation Service (WAS) is running in a sand-boxed mode. Disable the sand-boxed mode and it should resolve the issue
The following power shell script should do the job
$sa = Get-SPServiceApplication | where $_.TypeName.Equals("Word Automation Services")}
$sa.DisableSandbox = $true