wagtail_bakerydemo/bakerydemo/templates/blocks/image_block.html

{% load wagtailimages_tags %}

<figure>
    {% picture self.image format-{avif,webp,jpeg} fill-{400x220,600x338} sizes="(max-width: 768px) 200px, 900px" loading="lazy" %}
    <figcaption>{{ self.caption }} - {{ self.attribution }}</figcaption>
</figure>