Do you mean you want to override the default save method to put in some processing first? This is covered here
https://docs.djangoproject.com/en/dev/topics/db/models/#overriding-predefined-model-methods
If you are wanting to generate a legal URL slug, you can use the slugify method like this...
from django.template.defaultfilters import slugify
Class MyImageModel(models.Model):
def save(self, *args, **kwargs):
self.slug_field = slugify(self.some_unique_image_reference_field)
and then call the super save method as defined in the URL above.
HTH
Adrian
Auckland, NZ