models.py

class Photo(models.Model):
    thumbnail = models.ImageField(...)
    
    ...
    
    def get_small_thumbnail(self):
        return get_thumbnail(size='small')
    
    def get_medium_thumbnail(self):
        return get_thumbnail(size='medium')
    
    @property
    def large_thumbnail(self):
        return get_thumbnail(size='large')

serializers.py

class SinglePhotoSerializer(serializers.Serializer):
    small = DynamicMethodCharField(source='get_small_thumbnail', required=False, read_only=True)
    medium = DynamicMethodCharField(source='get_medium_thumbnail', required=False, read_only=True)
    large = CharField(source='large_thumbnail', required=False, read_only=True)

    class Meta:
        fields = ('small', 'medium', 'large',)

class PhotoSerializer(serializers.ModelSerializer):
    thumbnails = SinglePhotoSerializer(source='*', required=False, read_only=True)
  
    class Meta:
        model = Photo
        fields = ('thumbnails')