Random product thumbnails

The shop page on my website displays the "full-length" image of a product, and then, on hover, a "close-up" image.

I wish to randomise this, so that for some products, the "close-up" is displayed on load, and for others, the "full-length". And to have this random with every page load.

"Full-length" images are always named xxx1.jpg, and "close-up" images are xxx2.jpg, e.g.

<li>
    <div id="product">
        <img class="full-length" src="/image-1.jpg"/>
        <img class="close-up" src="/image-2.jpg"/>
    </div>
</li>

I guess I need a random number generator, between 1 and 2, and then for the second image to be what the first image is not.

Any help, sincerely and very much obliged.