utils.py

from io import BytesIO
from PIL import Image, ImageOps
from django.core.files.base import ContentFile
from stdimage.models import StdImageField, StdImageFieldFile

class StdRescaledImageFieldFile(StdImageFieldFile):
    def save(self, name, content, save=True):
        img = Image.open(content)
        file_format = img.format
        img.thumbnail(self.field.image_size)
        with BytesIO() as file_buffer:
            img.save(file_buffer, file_format)
            content = ContentFile(file_buffer.getvalue())
        super(StdRescaledImageFieldFile, self).save(name, content, save)

class StdRescaledImageField_(StdImageField):
    def __init__(self, rescale_image_down_to = None, *args, **kwargs):    
        super(StdRescaledImageField, self).__init__(*args, **kwargs)
        if rescale_image_down_to is not None:
            self.attr_class = StdRescaledImageFieldFile
            self.image_size = rescale_image_down_to