How to add new Foreign Key objects in a Form in Django?

I have a blog that uses Django and currently I am building a page for creating new posts.

In my Post model I have a Foreign Key field that is linked to a ImageField. When rendering the page using a generic class view, the field is displayed as a drop-down as expected, but I would like to add new thumbnails, not only selecting the existing ones, similar to the behavior of the admin page when there is a Foreign Key field, where:

1. There is a plus sign next to the drop-down
2. When clicked, shows a pop up where I can upload the image
3. After uploading the image, return to the main page, where the new option is created in the drop-down

How can I have a behavior similar to the admin's in my page? I tried to implement it as a widget, but the end result is very messy, as it just redirects to the admin page and I have to go back to my post creation page and reload to show the new option.

Alternatively I could just replace the the Foreign Key drop-down with a field to upload a file, but I don't know how, as the foreign key field expects an object not a file.

Here's my simplified models:

class Image(models.Model):
    image = models.ImageField()
    alt_tag = models.Charfield(max_length=30)

class Post(models.Model):
    title = models.CharField(max_length=60)
    content = models.TextField()
    thumbnail = models.ForeignKey(Image, on_delete=models.SET_NULL)

When rendering a page for creating new posts, I am using a generic class view:

class PostCreateView(CreateView):
    model = Post
    template_name = '../templates/post_create_form.html'