I’m not convinced that users understand that level of control. Opera Mini, for example, allows me to choose images on/ off, and (if on) low/ medium/ high quality images. I think that’s fine-grained enough; I don’t think most users have an understanding of how big 100K is, for example.
Also, that doesn’t account for the relative importance of some images. Imagine a photo-sharing site; the site’s logo is important to you as a business owner. The main image is important to the user; it’s what she came to see. If that were 110KB in size, the option you describe would block the main point of the page, and show a placeholder, while happily loading six 20K avatars of people commenters.
The lazyload and postpone attributes allow you to say “wait until after downloading the logo and main content image before downloading the avatars” or “don’t download any avatars in the comments list until they’re actually about be scrolled into view”, which is (I think) a better experience.