Hi @Dpa
Are you using Exchange On-premises or Exchange Online (Office 365)?
The built-in method to insert an image into emails is to use html to add the url of the image to the disclaimer.
Example, <IMG src="http://contoso.com/images/companylogo.gif" alt="Contoso logo">.
For recipients to see this image, they should be able to have access to this url.
Thus if this image is only accessible in internal network, external recipients may not see it correctly.
So to me it is not possible.
A workaround would be have users manually setup their signature in Outlook.
Or you may need to use some third-party solutions.
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