class CastImage(models.Model):
    image = models.ImageField(upload_to="castimages")
    cast = models.ForeignKey(Cast, blank=True, null=True, related_name='images')

class CastSerializer(serializers.ModelSerializer):
    images = serializers.RelatedField(many=True)
    class Meta:
        model = Cast
        fields = ('images',)

"results": [
        {
            "images": []
        },
        {
            "images": []
        },
        {
            "images": []
        } ]