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Ultimately I had to force the size of a uploaded image in the django admin area to a fixed dimen­sion of 620x250px. Even if it could look a simple thing, in fact it isn’t.

The main issue is that even if an Image­Field has a width_field/height_field option that refers to (pre­sum­ably) inte­ger fields that will be auto-​filled with the image size, we can’t ran a val­ida­tor across those fields (we can do so only in a form, but my prob­lem was to val­i­date the image in the admin area). So we have to man­u­ally load the image in memory and run a custom val­ida­tor that uses PIL to get the needed infor­ma­tion and val­i­date the image.

Here’s the result­ing code for the model:

class FImage(models.Model):
    ...
    img = models.ImageField(upload_to='images', blank=False,
            verbose_name=_("Image"),
            validator_list=[custom_validators.ImageLarge(620, 250,
            _("The image must be 620x250px large!")), ],
            help_text=_("Image must be 620x250px large."))
    ...

And for the val­ida­tor (that you should put in some sep­a­rated file):

from django.utils.translation import gettext_lazy as _
from django.core import validators
import StringIO
from PIL import Image

class ImageLarge(object):
    def __init__(self, width, height, error_message=_("The image isn't as large as expected!")):
        self.w, self.h, self.error_message = width, height, error_message

    def __call__(self, field_data, all_data):
        im = Image.open(StringIO.StringIO(field_data['content']))
        if im.size[0] != self.w or im.size[1] != self.h:
            raise validators.ValidationError, self.error_message 

The main issue with this solu­tion is that we store the entire image con­tent in memory for pro­cess­ing through PIL. Addi­tion­ally I don’t know if it will still work when the #2070 ticket’s patch will be included in trunk (and I didn’t checked with the
actual exist­ing patch).

If you want, you can read the full dis­cus­sion on django-​users.