Trying to get multiple elements to load with random positioning

So I'm not super great at JS yet, but I found this code block that does exactly what I need it to do - takes an element and makes it's position random. The problem is this only works if I use getElementById, but I want to do this with about 12 different elements, not just the one.

Ive tried giving them all the same class and using getElementsByClassName, I've tried using querySelector, I've tried querySelectorAll - With each one of these examples the code either doesn't work, or it only works for the first element.

Markup:

<div id="1" class="rando">
     <img src="someIMG" alt="logo" itemprop="image">
</div>

<div id="2" class="rando">
    <iframe width="200" height="250" src="someURL" frameborder="0" ></iframe>
</div>

JS:

function getRandomPosition(element) {
    var x = document.body.offsetHeight-element.clientHeight;
    var y = document.body.offsetWidth-element.clientWidth;
    var randomX = Math.floor(Math.random()*x);
    var randomY = Math.floor(Math.random()*y);
    return [randomX,randomY];
}

window.onload = function() {
    var rpos = document.getElementsByClassName("rando");
    rpos.setAttribute("style", "position:absolute;");
    document.body.appendChild(rpos);
    var xy = getRandomPosition(rpos);
    rpos.style.top = xy[0] + 'px';
    rpos.style.left = xy[1] + 'px';
}

I also tried putting these through a loop, but that didn't work either:

function getRandomPosition(element) {
    var x = document.body.offsetHeight-element.clientHeight;
    var y = document.body.offsetWidth-element.clientWidth;
    var randomX = Math.floor(Math.random()*x);
    var randomY = Math.floor(Math.random()*y);
    return [randomX,randomY];
}

window.onload = function() {
    var rpos = document.querySelector("#1, #2");
    for (var i = 0; i < rpos.length; i++) {
        rpos.setAttribute("style", "position:absolute;");
        document.body.appendChild(rpos);
        var xy = getRandomPosition(rpos);
        rpos.style.top = xy[0] + 'px';
        rpos.style.left = xy[1] + 'px';
    }
}

Any help / direction is, as always, greatly appreciated!

thanks!