var imgs    = new Array();
var imgcnt  = 0;
var thisimg = 0;

imgs[imgcnt++] = 'http://www.topforsalebyowner.com/10000459/pics/10000459a.jpg';

function nextpic() {
  if (document.images) {
    thisimg++;
    if (thisimg >= imgcnt) thisimg = 0;
    document.rollpic.src = imgs[thisimg];
  }
}

function prevpic() {
  if (document.images) {
    thisimg--;
    if (thisimg < 0) thisimg = imgcnt-1;
    document.rollpic.src = imgs[thisimg];
  }
}