Hi,
If you can't make local URLs accessible to Pdfcrowd service, try to save/render your local HTML to string or file and use convertString or convertFile API method. convertFile may contain an archive with all assets needed, so you can attach local images, CSS and JS also.
Alternative to the local tunnel is to enable access to local pages just for the Pdfcrowd conversion. It may be achieved by:
1. using a client certificate (API method setClientCertificate), e.g. for apache
https://www.google.com/search?channel=fs&client=ubuntu&q=apache+client+certificate2. using a custom HTTP header with some secret value (API method setCustomHttpHeader)
3. using a custom cookie with some secret value (API method setCookies).