Did you download them to a file or use the "inline" method: en.wikipedia.org/wiki/Data_URI_scheme ?
I download as a file, attached and computed the link. reasonably straightforward and safer than rely on some images on internet out of my control. Well, on client now they are fine, but some of them are not showing up on server - a routine investigation :-) Inline sounds clever, but having them as a file - they can be easier re-used for other purposes.
XPiNC (XPages in Notes Client) uses XULRunner (=embedded Firefox, unfortunately not the latest version) to render its content. It is rather restrictive in what it shows. There are some goatchas around ports, but that doesn't seem your problem here. You might need to investigate proxy settings. If everything fails, you could use a backend class to download the image and use the binary data (no need to save to disk) as image source.
thanks Stephan, I could not get to the bottom of it, but downloading and attaching the images instead seemed to be a good solution also from other considerations
well as it turns out java.net.URLEncoder encodes spaces as "+", so I still had to replace those manually... now it's tip-top