I haven’t tried this, but I’m relatively sure you can just set it as a default in your field.
pic = models.ImageField(upload_to='blah', default="path/to/my/default/image.jpg")
EDIT: Stupid StackOverflow won’t let me comment on other people’s answers, but that old snippet is not what you want. I highly recommend django-imagekit because it does tons of great image resizing and manipulation stuff very easily and cleanly.