For some reason, DomPDF won\'t render an image included in the html that is being parsed:
As there was another answer that suggests enabling the remote option in module.config.php
and I can't yet add comments, I thought it would be best to answer that this file does not exist in newer versions of DomPDF.
If you need to include remotely stored images in a newer version you have to pass it as an option to the constructor:
$dompdf = new Dompdf(array('enable_remote' => true));
This fixed the issue I had.