Display two images every X hours, Javascript

Below is a code I am running to display a pair of images each day. I have been unable to have it display the images randomly, but instead each day it selects the next image in the Array (not at random). Javascript is not my specialty and I have been unsuccessful in making it select a new random image in both arrays each day.

I have also been unsuccessful in making it select them every 12 hours instead of each day, which is what I would prefer it to do. It must display the same ones for every person who views it for that period, until the timer resets, if possible.

<script type="text/javascript"><!--
var imlocation = "http://ift.tt/2aSmrGm";
 function ImageArray (n) {
   this.length = n;
   for (var i =1; i <= n; i++) {
     this[i] = ' '
   }
 }
function linkArray (n) {
   this.length = n;
   for (var i =1; i <= n; i++) {
     this[i] = ' '
   }
}
image = new ImageArray(4);
image[0] = 'v85buoebv/Day2.png';
image[1] = 'djdl322kr/Evening2.png';
image[2] = 'arubcg423/Night2.png';
image[3] = 'xf9kiljm3/Morning2.png';

link = new linkArray(11);
link[0] = 'q4xda5xdn/CLOUDY.png';
link[1] = '7141ttkjf/Heavyrain.png';
link[2] = 'gzp0gatyz/lightrain.png';
link[3] = 'xc3njrxob/Medium_Rain.png';
link[4] = 'x0m770h8b/NO_WEATHER.png';
link[5] = 's38mlwf97/omgrain.png';
link[6] = 'btigj04l7/Special_Weather.png';
link[7] = 'b59m025vf/WEREALLGONNADIErain.png';
link[8] = 'ubmt38md7/Windy.png';
link[9] = 'x0m770h8b/NO_WEATHER.png';
link[10] = 'x0m770h8b/NO_WEATHER.png';
var currentdate = new Date();
var imagenumber = currentdate.getDay();
document.write('<div id="NOTICEME"><center><img src="' + imlocation + image[imagenumber] + '"><img src="' + imlocation + link[imagenumber] + '"></center><br><div id="mowords">[center][img]' + imlocation + image[imagenumber] + '[/img][img]' + imlocation + link[imagenumber] + '[/img][/center]</div></div>');
//--></script>

I used this code as a base to go on. But no matter how I toy with it, it won't give me a random image from the array. Also, the div ID "mowords" and "NOTICEME" is an ID I am using for CSS reasons that has nothing to do with this code.