TWO STEPS TO INSTALL A RANDOM IMAGE:

1. Copy the coding into the HEAD of your HTML document
2. Add the last code into the BODY of your HTML document

STEP ONE:
Paste the indented code into the <HEAD> of your HTML document

<SCRIPT LANGUAGE="JavaScript">

<!-- This script and many more are available free online at -->
<!-- The JavaScript Source!! http://javascript.internet.com -->

<!-- Begin
// Set up the image files to be used.
var theImages = new Array() // do not change this
// To add more image files, continue with the
// pattern below, adding to the array.

theImages[0] = '1.gif'
theImages[1] = '2.gif'
theImages[2] = '3.gif'
theImages[3] = '4.gif'

// do not edit anything below this line

var j = 0
var p = theImages.length;
var preBuffer = new Array()
for (i = 0; i < p; i++){
preBuffer[i] = new Image()
preBuffer[i].src = theImages[i]
}
var whichImage = Math.round(Math.random()*(p-1));
function showImage(){
document.write('<img src="'+theImages[whichImage]+'">');
}

// End -->
</script>

STEP TWO:
Copy the indented code into the <BODY> of your HTML document

<SCRIPT LANGUAGE="JavaScript">

<!-- This script and many more are available free online at -->
<!-- The JavaScript Source!! http://javascript.internet.com -->

<!-- Begin
showImage();
// End -->
</script>

The code for this comes from The JavaScript Source. The original page is at:
http://javascript.internet.com/miscellaneous/random-image.html