Back in the day, it was a big deal to make reasonable thumbnails of your images. You were rude not to.
Now, browsing without CSS, even the smallest icons on a webpage are very often huge images. :(
I don’t do much front end work these days but if I did, I would want a service that hosted all of your images and automatically resized them based on GET parameters, e.g. https://example.com/someimage.png?size=1024x768
We actually use Cloudinary at work and it does a lot of stuff based on the URL you pass it.
I use cloudinary for diving photos, e.g., this one. It loads a big, medium, or small image depending upon CSS media queries. I know they have more options, but the documentation is a bit chaotic. (The page itself is generated by sblg, which creates the links for me.)
I was responsible for porting some of our existing code at work to use Imgix. It worked great for us, simplifying a whole class of problems that came serving user-generate content. Reasonably powerful API (so dead simple for us to use: all query string driven). I particularly liked how it acts like a proxy, lazy loads and transforms the images.
It’s trivial, unless you care about performance (for many images).
There’s no mature, open source library for generalised GPU conversion (one of these days)…
Formats are inconsistently supported; third party images may be too large to fit in ram once decompressed (even if the file is small); I’m sure I’ve forgotten a few other issues…
In many cases (ok, not all) the site owner uploads the image. You’re not going to deliberately dos your own site with a bajillion pixel png. Caching is trivial, etc.
I don’t think it’s theoretical concerns that prevent this from being more commonplace. It’s just friction and intertia. e.g. bad defaults.
So, ImageMagick as a service? With caching, rate-limiting, etc? Yeah, I recall a friend of mine doing just that, like 5 years ago. Internally at his job, I mean. No idea if it’s still running.
The open source community already has this and it’s called Thumbor