I’ve been taking a closer look at how Discourse deals with image uploads. I think I have a reasonable grasp of the relvevant settings now. While experimenting with a 3MB image with a resolution of 4032x3024 I noticed that uploading via a computer resulted in an image downsized to 1920x1440 but when I uploaded the same photo from the Photos app on an iPhone it was stored the image at 4032x3024.
Why is there a difference between the desktop and mobile uploads?
Thanks for the speedy explanations. I’d made some progress towards understanding what is going on with the client-side optimisation but the blog post has brought it together. Every day is a school day.