I spent a few hours today reading through javascript documentation and stackoverflow questions to produce this: https://openuserjs.org/scripts/Arte....._Fit_to_Screen

About: A bit of javascript that seamlessly adds a 'Fit to Screen' button underneath submission images. Don't feel like scrolling around a particularly large image? Press the 'Fit to Screen' button to fit the image to your screen.

Note: Tested to work without issue on Firefox and Chrome with GreaseMonkey/TamperMonkey, tell me if anything goes wrong.

Update: Now works with both the classic and beta website layouts.