I think progressive jpeg works just fine, I recall it being quite popular in the early days of the internet when loading a large image could take a long time on

I think progressive jpeg works just fine, I recall it being quite popular in the early days of the internet when loading a large image could take a long time on landlines. You'd get a blurry preview that would incrementally become more detailed. I agree that it's niche these days because typically modern connection speeds mean that it's a waste of resources to iterate on the picture instead of loading it at once. Even on mobile networks the issues are connection reliability and latency, once the image starts pouring in it'll likely come in its entirety quite quickly. If you load images large enough that it becomes a real issue you would generally (in my experience) just have a smaller independent thumbnail/preview image you'll display while the large version is loading. That usually gives you more control on how things will look and avoid having a blurry mess on the user's screen.