Django ModelForm with extra fields

Question

So I have this ModelForm in my django project:

class DateForm(ModelForm):
    image = forms.ImageField()

    class Meta:
        model = Date
        exclude = ('user',)

the Photo model:

class Photo(models.Model):
    date = models.ForeignKey(Date, on_delete=models.CASCADE)
    image = models.ImageField(verbose_name='Photos', upload_to='media/date/photos/')

the form:

<p class="p-form">Title</p>
{{ form.title }}
<p class="p-form">Description</p>
{{ form.description }}
<p class="p-form">Place</p>
{{ form.place }}
<p class="p-form">Rating</p>
{{ form.rating }}
<p class="p-form">Photos</p> 
{{ form.image }}

Whenever I try to save my form, its form_invalid method is being called and it doesn't save the form. What is the matter? How do I save extra field of ForeignKey model? How can I get that images sent via form? Thanks!