If you want to use external images for intro or full article image, then you can just paste the image's url in that field.
Similarly, if you want to use external image inside your article body, you can use the insert/edit image button of the editor and paste the url of the image in the image source field.
However, although possible, it's not a recommended practice to use external images, unless the other site is yours.
First, you don't have control over those images, which can be deleted, changed anytime.
The owner of the other site can also restrict access to his files
It's not that ethical, as you are using resources and bandwidth of others.
You are creating many outgoing links to all other sites, which is bad for seo for your site.