$imgarray = array();
$dir = opendir (".");
while ($file = readdir($dir)) {
   if (!in_array($file, array('.', '..'))) $imgarray[] = $file;
}
echo $imgarray[rand(0,count($imgarray)-1)];

<SCRIPT LANGUAGE="JavaScript" TYPE="text/javascript">
.......
</SCRIPT>

<SCRIPT LANGUAGE="JavaScript">
var theImages = new Array()
// сюда добавляем картинки...
theImages[0] = '1.gif'
theImages[1] = '2.gif'
theImages[2] = '3.gif'
theImages[3] = '4.gif'
// ...
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]+'">');
}
</script>

<SCRIPT LANGUAGE="JavaScript">
showImage();
</script>

<img src="img1.gif" onMouseOver="this.src='img2.gif'" onMouseOut="this.src='img1.gif'">