[est]

s3 storage cost, html size and CDN traffic cost?

Cache one file to serve image of multiple sizes.

[rokkamokka]

But at that point, since you're not optimizing for filesize (everyone fetches every size), you might as well just provide the highest quality version only

[est]

> everyone fetches every size

Nah, by using the Content-Range header, you can download partition of the file and render lower resolution image.