[Answered ]-IntegrityError: may not be NULL

Actually you need to add null=True in the img field, So make it img = models.ImageField(upload_to='images', blank=True, null=True).
The difference is that because blank=True decides that whether the fields will be required by the forms. blank=False means that field cannot be blank and blank=True means field will not be required.
Regarding null=True sets NULL in column of your DB which will solve your issue.