function image(){

};

image= new image();
number=0;

//image array
image[number++]="<img src='random_images/random1.jpg'border='0'>"
image[number++]="<img src='random_images/random2.jpg'border='0'>"
image[number++]="<img src='random_images/random3.gif'border='0'>"
image[number++]="<img src='random_images/random4.jpg'border='0'>"
image[number++]="<img src='random_images/random5.jpg'border='0'>"
image[number++]="<img src='random_images/random7.jpg'border='0'>"
image[number++]="<img src='random_images/random8.jpg'border='0'>"
image[number++]="<img src='random_images/random9.jpg'border='0'>"
image[number++]="<img src='random_images/random10.jpg'border='0'>"
image[number++]="<img src='random_images/random11.jpg'border='0'>"
image[number++]="<img src='random_images/random12.jpg'border='0'>"
image[number++]="<img src='random_images/random13.jpg'border='0'>"
//keep item here

increment=Math.floor(Math.random()*number);
document.write(image[increment]);