At first, in your model make your fields empty and nullable:
class campoImagen(models.Model):
# [...]
imagen = models.ImageField(verbose_name='Imagen', upload_to='archivos', null=True, blank=True)
Then in your model form alter the required
parameter of your field:
class campoImagenForm(forms.ModelForm):
class Meta:
model = campoImagen
fields = ('imagen',)
def __init__(self, *args, **kwargs):
super(campoImagenForm, self).__init__(*args, **kwargs)
self.fields['imagen'].required = False
The model part is, however, obligatory. Otherwise you might be getting database inconsistency errors.
Keep in mind that blank and null are different things. They do, however, work well together and should give you what you want.