They do not reduce dimensions..but you can reduce bytes dramatically without losing quality.
I can take a 500,000 byte picture to 60,000 bytes.
It is more difficult to retain quality with layered graphics using this ..some work some don’t
300,000 bytes or less
http://tools.dynamicdrive.com/imageoptimizer/
over 300,000 bytes
JPG links only..no animations
http://www.321webmaster.com/optijpg.php
Oh! Then I had it backward. I assumed it reduced dimensions and did not compress, where it is the other way around.
Those numbers are excellent. That’s nearly 90% without losing quality.
You mean you are layering graphics by HTML, not software, right?