On the server
Configure your server to deliver cross domain images using headers that satisfy CORS restrictions:
On the client
Load the image using the crossOrigin flag set to anonymous:
var img=new Image();
img.crossOrigin="anonymous";
img.onload=function(){
...
ctx.drawImage(img,10,10);
}
img.src="yourImage.png";
If you want to test the client side before configuring your server, open a free account on dropbox.com and put your images in your public folder. Your public folder is CORS compliant.