JSFiddle - React, Tailwind, and code Playground

HTML

<input id="image-input" type="file">

JavaScript

var input = document.getElementById('image-input'),
    tag = document.getElementsByTagName('html')[0];

input.addEventListener('change', function() {

    var file = input.files[0],
        reader = new FileReader();
    
    if( !file ) {
        return;
    }

    reader.addEventListener('load', function( evt ) {

        // localStorage['background-image'] = reader.result;
        
        tag.style.backgroundImage = 'url(' + reader.result + ')';
        
    });
    
    reader.readAsDataURL(file);

});

/*if( localStorage['background-image'] ) {
    tag.style.backgroundImage = 'url(' + localStorage['background-image'] + ')';
}*/