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];
     }
     checkArrows();
}

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

function checkArrows() {
     if (thisPic < 1) document.arrowl.src = "images/arrowlg.gif";
     else document.arrowl.src = "images/arrowl.gif";

     if (thisPic > 1) document.arrowr.src = "images/arrowrg.gif";
     else document.arrowr.src = "images/arrowr.gif";
}

<SCRIPT>

<IMG SRC="images/pathfinder.gif" NAME="myPicture">

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

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