Пример работы (делаем масштабирование картинки по ширине и высоте, но не превосходящее width_max, height_max):

from django.db import models
from django.forms.fields import CharField
from imagekit.models import ImageSpec
from imagekit.processors import resize, Adjust
from django.forms import ModelForm, forms

class FormatImage(object):
    def process(self, image):

        # масштабируем до width_max, height_max
        width_max=605
        height_max=475
        c=resize.Fit(width_max,height_max,True)
        image=c.process(image)
        return image

class Photo(models.Model):
    original_image = models.ImageField(upload_to='photos')
    formatted_image = ImageSpec([FormatImage()],image_field='original_image', format='JPEG',
            options={'quality': 90},cache_to='photos/formatted/')
    thumbnail = ImageSpec([Adjust(contrast=1.2, sharpness=1.1),
            resize.Crop(50, 50)], image_field='original_image',
            format='JPEG', options={'quality': 90},cache_to='photos/thumbnail/')
    info=models.TextField()