Getting image width and height

- attach a function that gets the width and height of an image to the onload event for each images.
- once the images is finished loading, the function will be called, and we can get the values.
- 3 ways of attaching function to event:
myImage.setAttribute("onload", "getWH();");
myImage.onload = getWH;
myImage.addEventListener("load", getWH, true);

- we can pass parameters for the function only using the 1st method.
- loading animated gif requires longer time than still gif.