from sorl.thumbnail.fields import ImageWithThumbnailsField ,ThumbnailField

 imagen = ImageWithThumbnailsField(upload_to='images',
                                      thumbnail={'size': (75, 75)},
                                      extra_thumbnails={'icon': {'size': (25, 25),'options': ['crop', 'upscale']},
                                                        'large': {'size': (200, 400)},
                                                        },
                                    )

'sorl.thumbnail',

ImageWithThumbnailsField(..., generate_on_save=True)

class MyImage(models.Model):
    image = ImageWithThumbnailsField(upload_to='images',
                                      thumbnail={'size': (75, 75)},
                                      extra_thumbnails={'icon': {'size': (25, 25),'options': ['crop', 'upscale']},
                                                        'large': {'size': (200, 400)},
                                                        },
                                    )
    name= models.CharField(maxlength=100)