forms.py

from wagtail.images.forms import BaseImageForm
from .image_utils import resize

class ImageForm(BaseImageForm):
    # store user in __init__ and set default position of watermark
    def __init__(self, data=None, files=None, **kwargs):
        file = files and files.get('file', None)
        already_resized = kwargs.pop('already_resized', False)
        if not already_resized and file:
            new_file = resize(file, file.field_name, file.name, file.content_type)
            if new_file is not None:
                # if new size < 90% old sizes
                if new_file.size / file.size < 0.9:
                    files['file'].close()
                    files['file'] = new_file
                else:
                    files['file'].seek(0)
                    new_file.close()

        self.user = kwargs.get('user', None)
        super().__init__(data=data, files=files, **kwargs)

image_utils.py

import random
from io import BytesIO
from PIL import Image as PillowImage, ImageDraw, ImageFont

from django.core.files.uploadedfile import InMemoryUploadedFile

from wagtail.images.models import Filter, Image as WagtailImage

def resize(file, field_name, name, content_type):
    core_image = WagtailImage(file=file)
    # img = getattr(file, 'image', None)
    if core_image:
        image_filter = Filter(spec='max-4096x2304')

        new_willow = image_filter.run(core_image, BytesIO())
        # new_pillow = new_willow.get_pillow_image()

        new_file = InMemoryUploadedFile(
            new_willow.f,
            field_name,  # important to specify field name here
            name,
            content_type,
            # file.field_name,  # important to specify field name here
            # file.name,
            # file.content_type,
            len(new_willow.f.getvalue()),
            None,
            {}
        )
        new_willow.f.seek(0)
        # setattr(new_file, 'image', new_pillow)
        return new_file
    return None

settings__base.py

WAGTAILIMAGES_IMAGE_FORM_BASE = 'core.forms.ImageForm'