models.py

from django.db import models
from wagtail.wagtailimages.models import (
    AbstractImage,
    AbstractRendition,
    Image,
)


# We define our own custom image class to replace wagtailimages.Image,
# providing various additional data fields
class CustomImage(AbstractImage):
    alt = models.CharField(max_length=255, blank=True)
    credit = models.CharField(max_length=255, blank=True)

    admin_form_fields = Image.admin_form_fields + (
        'alt',
        'credit',
    )

    # When you save the image, check if alt text has been set. If not, set it as the title.
    def save(self, *args, **kwargs):
        if not self.alt:
            self.alt = self.title

        super().save(*args, **kwargs)


class Rendition(AbstractRendition):
    image = models.ForeignKey(
        'CustomImage',
        related_name='renditions',
        on_delete=models.CASCADE
    )

    class Meta:
        unique_together = (
            ('image', 'filter_spec', 'focal_point_key'),
        )