I set up an account too, and uploaded a photo so I could experiment.
If I click the ‘Share’ button, it generates a link, which works but has the usual error when pasted into Discourse:
If I click the image to open it fully and right-click to Copy image address, I get this, which doesn’t work at all (presumably because it’s not shared):
But if I paste the share link (first link in post) in a new tab, and click on the picture to open it, and then right click to Copy image address, I finally get the version with the groupsharetoken:
So that’s the workaround, but it’s a pain.
It seems like the proper share link method should work, but whatever it is providing is missing the ‘description’ tag Discourse is expecting.