I've got the following structure and JQuery function (based on [this](https://stackoverflow.com/a/17507894/2738738)):

```js
var sourceSwap = function () {
    var $this = $(this);
    var newSource = $this.data('alt-src');
    $this.data('alt-src', $this.attr('src'));
    $this.attr('src', newSource);
}

$(function() {
    $('img[data-alt-src]').each(function() { 
        new Image().src = $(this).data('alt-src'); 
    }).hover(sourceSwap, sourceSwap);
});
```

```html
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<img src="https://images.pexels.com/photos/414612/pexels-photo-414612.jpeg?auto=compress&cs=tinysrgb&dpr=1&w=500" data-alt-src="https://images.pexels.com/photos/237018/pexels-photo-237018.jpeg?auto=compress&cs=tinysrgb&dpr=1&w=500">
```

How would I go about adding a simple fade animation to it? At the moment it's just an instant transition