Show inline previews of images that are going to be uploaded.

previewImageUploads.js

// Licensed under a CC0 1.0 Universal (CC0 1.0) Public Domain Dedication
// http://creativecommons.org/publicdomain/zero/1.0/

(function (win,doc) {
    'use strict';
    if (!win.FileReader || !win.addEventListener || !doc.querySelectorAll) {
        // doesn't cut the mustard.
        return;
    }
    doc.querySelectorAll('input[type="file"][accept="image/*"]').forEach( function (fileInput) {
        fileInput.addEventListener('change', function () {
            var files = fileInput.files;
            if (files) {
                files.forEach( function (file) {
                    var fileReader = new FileReader();
                    fileReader.readAsDataURL(file);
                    fileReader.addEventListener('load', function () {
                        fileInput.insertAdjacentHTML(
                            'afterend',
                            '<img src="' + this.result + '">'
                        );
                    });
                }
            }
        });
    }
}(this, this.document));

Blog post: Image previews with the FileReader API.

You should be able to drop this script into any page that has:

<input type="file" accept="image/*">

Closing parenthesis are missing on lines 23 and 26 :)