Re: pre-scaling web images to avoid bandwidth bloat
how about Image Magick's 'convert' on the server side?
Bit of a sledgehammer for a walnut really and too slow for constant use – don't forget you're going to have manage names.
But everything is available as mod_pagespeed (exists fro Apache, Nginx, IIS), which can handle caching and has some heuristics for content-negotiation / bandwidth, because you may well want at least four different versions of the same image for mobile, desktop, hi-res, JPEG, WEBP, etc.
In my view, the complexity associated with getting this right takes it outside the web "coders" responsibility and into infrastructure. Nearly all the attempts of web developers to solve these problems have led to, at best, half-baked solutions.