Despite URL appearances, https://www.dropbox.com/s/mqprfsnxghdtbhx/1083.jpg is not actually an image URL – it’s a web page.
If you visit that page then right click the image, you’ll find the following image URL:
https://uccb66a3aee9072acd9b699196bd.previews.dropboxusercontent.com/p/thumb/AA9ki-uSihB0rwmq4g8QsI2SOOjemmZnEF7nFHj_2m2vRS2mQHxZkcz5d3q–Cpt_PNnR7XQCMGq1IqGzSKAw-Vrfb1uq3H1jHIAyXk9MKT3UPDlaDQqSYjC1dORcj8xsxCpDyaYVxd_LtBx_E_8-rmOwn0UUqtoOCTlyuTXproxvZsWJrEkKAp6u4J3S9wZ20cC6K_LJJDIMOAYB0wttlXJu24yoMPZaJdi427oxNKDlKxZF2XnNWmPJjFhb_j64eFZ5atbkD3YtnhTJv4JtEL0NtrGhoG0JUVV_vrGZw92f21MzoyEYdOyan682MiHwXUKgQLEg_sKxQ5giKDsYOy86SEvxwxdDflTTzO1pONYdKOyD8OH-EFawoV6TorcC7IEvhLBb-dLN7z1ZuvpK2-S/p.jpeg?fv_content=true&size_mode=5
That image URL should work in External Images.
Alternatively, if you change ?dl=0 to ?dl=1 then that should probably work too:
https://www.dropbox.com/s/mqprfsnxghdtbhx/1083.jpg?dl=1
Edit: Also – some extra info – Dropbox don’t like to be used as a CDN, similarly with Google Drive. It actually goes against their terms of service. It won’t matter unless/until you get a lot of traffic, but just a word of warning for using them as your CDN.
Also – this page here says change the URL parameter from ?dl=0 to ?raw=1 and then it should just be the image URL and that should work for you.
https://www.dropboxforum.com/t5/Dropbox-files-folders/Can-I-use-Dropbox-as-a-CDN/td-p/278852
But please heed the warning on that page:
[dropbox] share links have a limited amount of bandwidth per day, and if you’re linking to large files it could be very easy to exceed that limit and have ALL of your links banned temporarily. Simply put, while you can serve files out of Dropbox, it is not a CDN and you probably shouldn’t use it as one.