calibre-web/cps/templates/book_cover.html

{% macro book_cover_image(book, thumbnails) -%}
    {%- set book_title = book.title if book.title else book.name -%}
    {% set srcset = thumbnails|get_book_thumbnail_srcset if thumbnails|length else '' %}
    {%- if srcset|length -%}
        <img
            srcset="{{ srcset }}"
            src="{{ url_for('web.get_cached_cover', cache_id=book|book_cover_cache_id) }}"
            alt="{{ book_title }}"
        />
    {%- else -%}
        <img src="{{ url_for('web.get_cached_cover', cache_id=book|book_cover_cache_id) }}" alt="{{ book_title }}" />
    {%- endif -%}
{%- endmacro %}