Follow these steps to apply a watermark to the displayed idea images:

1. If you haven't done so already, install django-imagekit into your virtual environment:

(env)$ pip install django-imagekit==4.0.2

2. Put "imagekit" into INSTALLED_APPS in the settings:

# myproject/settings/_base.py
INSTALLED_APPS = [
    # …
    "imagekit",
]

3. In the core app, create a file called processors.py with a WatermarkOverlay class, as follows:

# myproject/apps/core/processors.py
from pilkit.lib import Image

class WatermarkOverlay(object):
    def __init__(self, watermark_image):
        self.watermark_image = watermark_image

    def process(self, img):
        original = img.convert('RGBA')
        overlay = Image.open(self.watermark_image)
        img = Image.alpha_composite(original, 
        overlay).convert('RGB')
        return img

4. In the Idea model, add the watermarked_picture_large specification next to the picture field, as follows:

# myproject/apps/ideas/models.py
import os

from imagekit.models import ImageSpecField
from pilkit.processors import ResizeToFill

from django.db import models
from django.conf import settings
from django.utils.translation import gettext_lazy as _
from django.utils.timezone import now as timezone_now

from myproject.apps.core.models import CreationModificationDateBase, UrlBase
from myproject.apps.core.processors import WatermarkOverlay

def upload_to(instance, filename):
    now = timezone_now()
    base, extension = os.path.splitext(filename)
    extension = extension.lower()
    return f"ideas/{now:%Y/%m}/{instance.pk}{extension}"

class Idea(CreationModificationDateBase, UrlBase):
    # …
    picture = models.ImageField(
        _("Picture"), upload_to=upload_to
    )
    watermarked_picture_large = ImageSpecField(
        source="picture",
        processors=[
            ResizeToFill(800, 400),
            WatermarkOverlay(
                watermark_image=os.path.join(settings.STATIC_ROOT, 
                'site', 'img', 'watermark.png'),
            )
        ],
        format="PNG"
    )

5. Using a graphical program of your choice, create a semi-transparent PNG image with white text or a logo on a transparent background. Make it 800 x 400 px in size. Save the image as site_static/site/img/watermark.png. Here's what it might look like:

6. Run the collectstatic management command afterward:

(env)$ export DJANGO_SETTINGS_MODULE=myproject.settings.dev
(env)$ python manage.py collectstatic

7. Edit the idea detail template and add the watermarked image there, as follows:

{# ideas/idea_detail.html #}
{% extends "base.html" %}
{% load i18n %}

{% block content %}
    <a href="{% url "ideas:idea_list" %}">{% trans "List of ideas" 
     %}</a>
    <h1>
        {% blocktrans trimmed with title=idea.translated_title %}
            Idea "{{ title }}"
        {% endblocktrans %}
    </h1>
    <img src="{{ idea.watermarked_picture_large.url }}" alt="" />
    {{ idea.translated_content|linebreaks|urlize }}
    <p>
        {% for category in idea.categories.all %}
            <span class="badge badge-pill badge-info">
             {{ category.translated_title }}</span>
        {% endfor %}
    </p>
    <a href="{% url 'ideas:download_idea_picture' pk=idea.pk %}" 
     class="btn btn-primary">{% trans "Download picture" %}</a>
{% endblock %}