To insert an (either externally hosted, or previously uploaded by you to FPN's server) image by URL:
Click on the Other Media button in the bottom right corner of the WYSIWYG editor panel.
Select ‘Insert image from URL’ on the pop-up menu.
Type or paste the URL into the thus-named field in the pop-up dialog box, and click on ‘Insert into post’.
(In this example, I inserted an image that is hosted on Google Photos.)