When referencing an external image in a Visualforce PDF rendered image, the image is displaying a broken link. It works perfectly without the
I just figured this out after 30 minutes of Googling and thought a concise post may help others in future. It was not documented in the SFDC documentation but was on an different post from 2012.
The issue was that the PDF rendering engine runs under the same security restrictions as Apex. In other words, the external URL must be allowed in the remote site settings to be used by the renderer.