Unfortunately, you can't display image attachments inside the popup window. What you can do, as you found in the doc, is you can check the box when configuring the popup window to display links that represent your attachments (see attached image). When you click on this link in the popup window, the browser will download the attachment to your computer.
We do want to support the ability to view attachments in the popup window. We just haven't done so yet.
Thanks,
Mike