Hey, this is an old thread, but if others have this issue, I had this broken image in PDF templates and was able to fix it:
-make sure the image is in a writeable directory on the same server as your SuiteCRM install, not a link to an external website/ server (eg localhost/uploads/)
-make sure the image itself has the correct permissions. I’ve reapplied permissions countless times on our server, but a logo uploaded via FTP still had the default 644 file permissions. Change permission on the image to 775.
-use a relative path in the HTML link to the image, eg 
Hope that helps.