var myPix = new Array("images/pathfinder.gif", "images/surveyor.gif", "images/surveyor98.gif");
var thisPic = 0;

function processPrevious() {
     if (document.images && thisPic > 0) {
          thisPic--;
          document.myPicture.src = myPix[thisPic];
     }
}

function processNext() {
     if (document.images && thisPic < 2) {
          thisPic++;
          document.myPicture.src = myPix[thisPic];
     }
}

<SCRIPT>

<A HREF="javascript:processPrevious()">
<IMG SRC="images/arrowl.gif" ALT="Previous">
</A>

<A HREF="javascript:processNext()">
<IMG SRC="images/arrowr.gif" ALT="Next">
</A>