Can't add x and y member variables into an Image class

I created a new image object with a call,

var myImage = new Image();

I initiate it like so,

myImage .src = "./png/iso_tile2.png";
myImage .thisImage = myImage ;

As seen above, I added .thisImage variable as reference to "this" so I can plug in width and height of image as member variable to this Image object.

I did that in onLoad() function,

myImage .onload = function() 
{                       
this.width = this.naturalWidth;
this.height = this.naturalHeight;

this.x = 25;
this.y = 15;

}

As you can see, I also added x and y as new member for me to hold its position in the screen I'm drawing it. I set them to 25, 15 respectively.

But when I access them, I always get 0, 0. If i change their name to say _x, _y then it works. So I can only assume that x, y are private members of Image class?