Okay, I get what you're saying. For the record, WE didn't make these changes. This is simply the latest version of vBulletin, and "every other forum on the internet" that is using the latest version of vBulletin is suffering the same problems and learning curve.
Actually, it appears that the functionality you want is there. Click the image icon on the toolbar, at the very top of that popup you can select "from computer" or "from URL". Select "from URL". Paste your URL, uncheck "retrieve file" (it will upload the image as an attachment and thumbnail it if you don't!), click "OK". Done.
I just tried it below. Perfect IMG tag in either the WYSIWIG editor or Source Mode editor.
Try it. Let me know if there are issues. Seems to work once you understand how it works, and it's simple enough. (I have to admit, I don't like the new file upload selection nonsense, but it works, too)