var image = "images/pic-main-";
var ext = ".jpg";

var captions = new Array();
captions[1] = "Some kind of nice statment here that in a sentence or so sums up what we do so the surfer checks the page and says, yep, I want to spend some time here looking around, because the picture of those keys is so neato.";
captions[2] = "Here is another sentance or two about some other important thing or another.  This picture of a lightbulb really makes the visitor want to stay and look around some more.";
captions[3] = "Another great message about this wonderful website and the wonderful products found within.  A great way to intrigue site visitors and get them to look around some more is surely with a typewriter ball.";

var rand = 1;

function pickImage() {
  rand = (new Date).getTime() / 1000;
  rand = Math.floor(((rand - Math.floor(rand)) * (captions.length-1)) + 1);
  image += rand+ext;
}

pickImage();