form.py

import urllib
from StringIO import StringIO
from django import forms
from django.core.files import File


from formexemplo.models import Pessoa


class PessoaModelForm(forms.ModelForm):
    img_link = forms.CharField(max_length=250)


    def save(self, commit=True):
        pessoa = super(PessoaModelForm, self).save(commit=False)
        url = self.cleaned_data['img_link']
        pessoa.foto.save('teste.jpg', File(open(self.download_foto(url))))
        if commit:
            pessoa.save()
        return pessoa

    def download_foto(self, url):
        resp = urllib.urlretrieve(url)
        return resp[0]

    class Meta:
        model = Pessoa
        fields = ['nome']

models.py

from __future__ import unicode_literals

from django.db import models

# Create your models here.


class Pessoa(models.Model):
    foto = models.ImageField(upload_to='images/')
    nome = models.CharField(max_length=50)