I'm a new Joplin user, though with experience in Markdown.
Manjaro Linux desktop client, Version 1.7.11. Android current to Google Play version update 3 Feb 2021.
I can't create an image link to an image on my Dropbox account. Pasting test images into the same document (Joplin logo and image from the welcome file) both work.
I have gone to Dropbox in Firefox and the Android app, copied the link to the image, pasted it into the Markdown image link. I get a broken link. If I edit it to a regular link, it works.
Synchronization between all three devices, Linux workstation, Chromebook is enabled and working smoothly.
Apologies if I am missing something obvious, but I have googled and searched the forums here and can find no solution.
Thanks for replying. All help is gratefully received.
The image link to the Dropbox image looks like this: 
If I edit it by removing the exclamation mark, it works as a link and opens the image in Chrome.
This link, to an image on a NASA webpage, works perfectly. 
The Dropbox link is copied using the Android Dropbox app.
It looks like that URL isn't just image, at least when I open it in browser I get some popup and all sorts of other UI elements.
I suppose this is why Joplin can't embed it.