Hi,
You can view the parallax scroll here --->
www.chrislongley.co.uk/
I do not use the 100% module width because I do not set wide website but boxed. That is how I prefer it. But it still works responsive design in whatever module width.
But for people who do force the wide module you need to create the image (height of module x60% pixels) x 1920 pixels to get the best resolution in the module area. If you just used a boxed website width say of 1200 pixels and a module height of 400 then a background image 1200 x 640 will work nicely. Yoy have to do some experimentation of positioning of background and png images!
Unfortunately the scroll does not work in IE8. The png images just float. I will test in IE9/10 soon.
If anybody wants some custom artwork then get in touch.