If it is a firewall or network issue then image the should not be visible when pasted the link on the web browser.
But when i paste image url on to the web browser address bar, i can view the image. Also I am able to view the image with the same URL in my web application page(html page).
May be we need to check how pd4ml jar treats the image resource Url. And in which all cases pd4ml.jar throws this error.
Let me know if anyone has more information to share in this case.