image randomly changes after a certain interval

i tried this, but still it is not changing:

var images = []
images[0] = "pics/1.jpg";
images[1] = "pics/2.jpg";
images[2] = "pics/3.jpg";
images[3] = "pics/4.jpg";
images[4] = "pics/5.jpg";
setInterval("changeImage()", 30000);
var x = 0;
function changeImage(every_seconds) {
if (x < 5) {
document.getElementById("pics").src = images[x]
x++;
}
if (x == 5) x = 0;
}