JavaScript change img src attribute without jQuery

try this...hope it works

window.onresize = window.onload = function () {
    if (window.innerWidth > 1536) {
        var logo = document.getElementById('rm');
        logo.setAttribute('src','img/rm2.png');
    }
};

You mean you want to use pure javascript?

This should do it:

var logo = document.getElementById('rm');
logo.src = "img/rm2.png";

So your function should look like :

window.onresize = window.onload = function () {
    if (window.innerWidth > 1536) {
      var logo = document.getElementById('rm');
      logo.src = "img/rm2.png";
    }
};

Note: You could also use element.setAttribute. BUT, see this post for more:
When to use setAttribute vs .attribute= in JavaScript?