If I understand you correctly, you propose a image format which should provide a better quality for less file size. However, to implement an in-browser client-side reader, one probably needs a javascript library, and this will blow up the size again [Side note: Actually I am surprised the site uses JavaScript at all, being that static].

Another proposal for image compression is to fall back on SVG, c.f. https://jmperezperez.com/svg-placeholders/ -- at least this does not need JS.