Django widget to replace the file selection standard button by an image preview. Tested with Django 1.8.5

forms.py

from django.forms import ModelForm, FileField

from .models import Campaign
from .widgets import ImagePreviewWidget


class DesignCampaignForm(ModelForm):

    brand_logo = FileField(widget=ImagePreviewWidget)

    class Meta:
        model = Campaign
        fields = ['brand_logo', 'brand_description']

image-preview-widget.css

/* Hide the input element for the image-preview widgets.
The "open file" dialog will be triggered by a click on the image preview */
input[type="file"].image-preview {
    display: none;
}

image-preview-widget.js

function ImageRefresher() {
};

function init(inputId) {
    var that = this;
    this.inputId = '#' + inputId;
    this.imgId = '#' + inputId + '_img';
    this.origData = $(this.imgId).attr('src');
    $(this.inputId).change(function(){
        that.readURL(this);
    });
}

function readURL(input) {
    var that = this;
    if (input.files && input.files[0]) {
        var reader = new FileReader();
        reader.onload = function (e) {
            $(that.imgId).attr('src', e.target.result);
        }
        reader.readAsDataURL(input.files[0]);
    } else {
        $(this.imgId).attr('src', this.origData);
    }
}

ImageRefresher.prototype.init = init;
ImageRefresher.prototype.readURL = readURL;

window.onload = function() {
    $('.image-preview').each(function(index) {
        var refresher = new ImageRefresher();
        refresher.init(this.id);
    });
};

widgets.py

from django.forms.widgets import ClearableFileInput, Input, CheckboxInput
from django.utils.safestring import mark_safe
from django.utils.html import conditional_escape, format_html, html_safe


class ImagePreviewWidget(ClearableFileInput):

    '''Basically a ClearableFileInput widget, but:
         - remove from the template most clutter: we leave only the image, the clear and the <input>
         - we define a class for the input, so that we can hide it with CSS
    '''

    # The "for" in the <label> allows to open the "open file" dialog by clicking on the image, no js involved
    template_with_initial = (
        '<label for=%(id_for_label)s><img id="%(img_id)s" src="/media/%(initial)s" width="100px"></label>'
        '%(clear_template)s<br />%(input)s'
    )

    INPUT_CLASS = 'image-preview'  # This is the class of the <input> element, which we want to hide

    def __init__(self, attrs=None):
        super(ImagePreviewWidget, self).__init__(attrs)
        self.attrs['class'] = self.INPUT_CLASS

    # Override ClearableFileInput:render
    def render(self, name, value, attrs=None):
        id_for_label = self.id_for_label(attrs.get('id'))
        substitutions = {
            'initial_text': self.initial_text,
            'input_text': self.input_text,
            'clear_template': '',
            'clear_checkbox_label': self.clear_checkbox_label,
            # We need the id, so that clicking in the image triggers the "Open file" native window
            'id_for_label': id_for_label,
            #
            'img_id': id_for_label + '_img',
        }
        template = '%(input)s'
        substitutions['input'] = Input.render(self, name, value, attrs)  # call Input.render directly

        if self.is_initial(value):
            template = self.template_with_initial
            substitutions.update(self.get_template_substitution_values(value))
            if not self.is_required:
                checkbox_name = self.clear_checkbox_name(name)
                checkbox_id = self.clear_checkbox_id(checkbox_name)
                substitutions['clear_checkbox_name'] = conditional_escape(checkbox_name)
                substitutions['clear_checkbox_id'] = conditional_escape(checkbox_id)
                substitutions['clear'] = CheckboxInput().render(checkbox_name, False, attrs={'id': checkbox_id})
                substitutions['clear_template'] = self.template_with_clear % substitutions

        return mark_safe(template % substitutions)


    class Media:

        css = {
            'all': ('css/image-preview-widget.css',)
        }
        js = ('js/image-preview-widget.js', )