Thank you for contacting Syncfusion support.
We have checked the reported image missing issue, but the image is converted properly. We have attached the input HTML file with provided image URL and output PDF document which is converted using WebKit converter settings. Please find the document below,
We suspect that the reported image missing issue may occurs due to missing of OPENSSL assemblies in the machine where the conversion takes place. WebKit rendering engine requires OPENSSL assemblies for accessing the webpage/resource from HTTPS URL. So, kindly try the conversion with OPENSSL assemblies and let us know the result. Please refer below links for more details about the OPENSSL assemblies.