Seems all the answers can be found on Reddit! When we were talking about this over in Discord, I wasn’t aware this was more difficult than it used to be. Anywho, the TL;DR is to go to your Followers page & edit it at the top there.
I never would have thought to scroll up on that page.
The new image size, 1200x380, is very awkward.
Edit - Actually I can’t get any of my images to fit in the box.
But you know where that image size does not look awkward?
I just cropped an image that @Vocino made for me and it looks really good on my tablet. I might try another.
Edit again - I don’t think this new banner is meant to be seen on browser. I think that’s why they hid it on the followers page (it is actually on the following page also) as they did.
If you click on anyone’s avatar with the mobile app then you can easily see their banner image.