This line checks to see if the user’s browser will understand the image object, which means that it has the ability to manipulate images via JavaScript (which is what we’re trying to accomplish).

Lots of high-res images can really spruce up a Web site. But they can also slow it down-- images are files, files use bandwidth, and bandwidth is directly related to wait times. It's time you get ...

Remember that because we named our image myButton, we created a new image object document.myButton. In order to change the image that’s displayed, we need to update the src property of this image.