External Images work everywhere that allows featured images and/or galleries.
With the new WordPress Gutenberg editor, the external image box has moved from the right hand side to the bottom of the page. Here’s an example for a new article:
Adding an external featured image
To add a featured image, paste the URL into the top box in the URL above. You can also paste multiple images in here if you wish, separated by commas, and the first will be used as the featured image.
Deleting your external image
To delete the external image, and return to using the built-in WordPress featured image, simply empty the contents of the text box.
Adding a gallery to a post/custom post
Paste the additional images into the gallery box (2nd box above), separated by commas. Alternatively, you can simply list all your images, separated by commas, in the top box. If you add images to both boxes, they will be de-duplicated before the gallery is constructed.