This link should get you started with regards to creating rounded corners in a HTML 5 canvas.
http://www.html5canvastutorials.com/tutorials/html5-canvas-rounded-corners/
It will also allow you play around and come up with a solution that looks like your image. With regards to then putting this behind other elements, wrap the remainder of your elements in a div and apply
z-index: 10
to this div (ensuring this is a higher number than the z-index applied to the canvas element). Refrain from using
z-index: -1
as this can cause havoc in some browsers.
See here for more info:
An html5 canvas element in the background of my page?
Obviously using a HTML5 canvas means that your site would not be fully backwards compatible.
See here for info regarding compatibility:
http://caniuse.com/#feat=canvas
N.B. I can’t really help you anymore than this as I’ve never used HTML 5 canvas. I personally think it’s an over complicated way of presenting an image but I guess others have a more practical use for it.
solved Draw vector shapes for background of page