Example:
You have 4 images:
image1.jpg
image2.jpg
image3.jpg
image4.jpg
put 'em all in an array like this:
$Array[0] = "image1.jpg";
$Array[1] = "image2.jpg";
$Array[2] = "image3.jpg";
$Array[3] = "image4.jpg";
then, initialize the seed of the random number generator like this:
srand(microtime());
and then, use rand to get a number between 0 and 3, like this:
$value = rand(0,3);
And then, use a simple HTML to show the image:
<IMG SRC="<? echo $Array[$value] ?>">
Hope that was it. There are other ways, but this is fast and clean.
fLIPIS