Possible bug I need to test tonight but might be a particular to my setup!
I was messing around with a "private" site using the Members plugin (enabling the option "Redirect all logged-out users to the login page ... "
I noticed that all the images in the PDFs generated from private site were missing.
If I used relative URLs for the images then they appeared in the PDF. I'm using this plugin here as a workaround.
(src="/wp-content/uploads/2013/12/lightning.jpg" instead of src="http://gunyon.co.uk/wp-content/uploads/2013/12/lightning.jpg"
Your plugin is not alone in with this issue... mpdf also has a problem with missing images. So it's maybe the site's fault rather than the plugin.
I have some more info tonight! this is just a note