I have been having the same problem with a logo image not showing up in the PDF that is created as a form completion event... (what's strange is that there was a time in the middle of September when it did show in the PDF even though no settings have been changed that I remember!)
I am using the Telerik editor... On the site that isn't working, it is a standard
www.site.com URL... but on my sandbox environment, which is
www.site.com/other, the image is actually showing up in the PDF. In the one that worked, it was a relative path without the
www.site.com portion, but neither that nor the full path works in the other site. I also tried a base href but unfortunately, that did not make any difference...