I'm gonna say something similar to that I said the last time someone requested this:
You can already make quite high-quality stuff with just 1920x1080 (i.e. the Full HD pixel amount), in fact my pfp shows an example, and that's just barely 2MB as a PNG file. JPGs like the kind your phone takes are even better, with about 8 million pixels, 3200x2400, at just over 2MB. Many people also have not much bandwidth and can't handle huge images like that.
I do dislike it when I do have to upload smaller images, but it's just so rare for me. Maybe I don't relate as much as I should, but I don't think it's necessary. Convenient? Yes. A good incentive to buy premium? Yes. But personally I don't see the necessity of this.
I think that if this is added then there should also be an option to turn Big Images (i.e. >4MB) off, even without premium, and when Big Images are loaded with the setting off, then it uses a lower resolution version, kinda like how avatar images work most places - it would be easy enough to just generate a low-res version of the file using a script such that the image is below 4MB again.