To insert images with the simple image code, you click the Insert Image button (the tree in the picture frame). Let it load fully. Now click the tab that says From URL.



Untick the box that says Retrieve remote file and reference locally (I think this means that the file will be downloaded to the PXR server and then uploaded as an attachment, rather than directly linked from the URL, however it didn't work when I tried it). Then you will simply get given the original URL in image tags.

Honestly I never use those buttons anyway, because it's so much easier just to type [img][/img]. But I can see how it would be pretty annoying. I'm not sure if there's a way to simplify it. Let's wait and see if anyone already knows a simple solution, otherwise I'll see what I can find!