A simple way to do this is as follows:

Name all the pictures with the same name but different number extension: img_1, img_2, img_3...etc.

Define a function that randomly create a img (src) name. For example, if you have 10 images:

The HTML code:

<img id=imageid src=>

And the JS code:

function randomImg(){

var randomNumber = Math.floor(Math.random() * 10) + 1;

var imgName = img_ + randomNumber;

document.getElementById(imageid).src= YOUR_IMG_PATH + / + imgName ;

}

Hope it helps!

EDIT: If you have a .jpg or .png image, make sure to include that in the function or it won't work. For example for a JPG image:

function randomImg(){

    var randomNumber = Math.floor(Math.random() * 12) + 1;

    var imgName = img_ + randomNumber + .jpg;

    document.getElementById(greenbox).src= YOUR_IMG_PATH + / + imgName ;

 }