lb-

from wagtail.wagtailforms.forms import FormBuilder
from wagtail.wagtailimages.fields import WagtailImageField

class ExtendedFormBuilder(FormBuilder):
    def create_image_upload_field(self, field, options):
        return WagtailImageField(**options)
    FIELD_TYPES = FormBuilder.FIELD_TYPES
    FIELD_TYPES.update({
        'image': create_image_upload_field,
    })

lb-

from wagtail.wagtailforms.models import AbstractFormField, FORM_FIELD_CHOICES
class FormField(AbstractFormField):
    FORM_FIELD_CHOICES = list(FORM_FIELD_CHOICES) + [('image', 'Upload Image')]
    field_type = models.CharField(
        verbose_name=_('field type'),
        max_length=16,
        choices=FORM_FIELD_CHOICES)
    page = ParentalKey('FormPage', related_name='form_fields')