My very basic understand of Discourse (and I am very much a novice) is that uploaded images are stored on your site, and an optimised file size version of the image are also stored. Initially, the original image will be displayed to users, and at some point, the forum switches over to an optimized image (to me that appeared to be after 24 hours).
I experimented a little bit with the settings as I was concerned about running out of storage.
On your admin dashboard, check them out (Under Files section). There are various settings to control image size, and dimensions. This might be related to your rogue image problem (if I understand it correctly). For example, your reformated image still falls outside the criteria.
My issue when I transferred hosts from Digital Ocean To Hetzner was that I had some images (and avatars) that simply were not appearing (similar to your first screenshot). My solution was FTP move them (crude but it worked). Not 100% sure why. At first I moved only the optimized images and it didn’t fix the problem, but when I moved all the images it resolved my issue.
Apologies about folder mistake I was doing it from memory. The folder structure is: VAR/Discours/Shared/Standalone/Upload
Its very easy to login with an FTP client with Digital ocean
Host ; IP address of Discourse
Port : 22
Username: root
Password: your SSH password
As I said I am far from an expert, more someone that has tinkered with it the last couple of months.
There are people here that can offer better advice (and insight). But if all else fails it might be worth giving my approach a go.