Hello. I am looking at the following tutorial:

<!DOCTYPE html>
<html>
<body>
<script>
function changeImage()
{
element=document.getElementById('myimage')
if (element.src.match("bulbon"))
{
element.src="pic_bulboff.gif";
}
else
{
element.src="pic_bulbon.gif";
}
}
</script>

<img id="myimage" onclick="changeImage()"
src="pic_bulboff.gif" width="100" height="180">

<p>Click the light bulb to turn on/off the light</p>

</body>
</html>

What I don't understand is this line:
element=document.getElementById('myimage')

Where does "myimage" come from? Is "myimage" the name of a file? Is "myimage" the name of an object that I have to define somewhere? I assume that "pic_bulboff.gif" is the name of a file that should be kept in the same directory as the web page.

However, I would think the tutorial would do a better job of explaining where all these things come from. Anyone know of a better tutorial?