boards/static/js/form.js

var isCompact = false;

$('input[name=image]').wrap($('<div class="file_wrap"></div>'));

$('body').on('change', 'input[name=image]', function(event) {
    var file = event.target.files[0];

    if(file.type.match('image.*')) {
        var fileReader = new FileReader();

        fileReader.addEventListener("load", function(event) {
            var wrapper = $('.file_wrap');

            wrapper.find('.file-thumb').remove();
            wrapper.append(
                $('<div class="file-thumb" style="background-image: url('+event.target.result+')"></div>')
            );
        });

        fileReader.readAsDataURL(file);
    }
});