If you open the HTML in a web browser, web browser establishes a connection to load HTML itself and establishes connections to load referenced images.
When you request Weblogic to generate a PDF, your web browser establishes a connection to load PDF, but connections to load images are initiated by Weblogic.
Usual problem reason: Weblogic server has no routing to the image locations. Please make sure the server, where Weblogic runs, can resolve the image location addresses and has a routing to the image server.
A switching of debug info on (pd4ml.enableDebugInfo()) and analyzing Weblogic’s log can help.