If the plus sign makes addition, then the minus sign must make subtraction. Look at the code below. No need to rearrange the images, although you could. Just set it so that num loses one each time and when you get to zero, start again at three.
<html>
<head>
<SCRIPT type="text/javascript">
num=4
img1 = new Image ()
img1.src = "pic1.gif"
img2 = new Image ()
img2.src = "pic2.gif"
img3 = new Image ()
img3.src = "pic3.gif"
function slideshow()
{
num=num-1
if (num==0)
{num=3}
document.mypic.src=eval("img"+num+".src")
}
</script>
</head>
<body>
<center>
<img src="pic1.gif" name="mypic" border=0>
<p>
<a href=
"JavaScript:slideshow()">Click for next picture</a>
</center>
</body>
</html>