calibre-web/cps/templates/book_cover.html

{% macro book_cover_image(book, title=None) -%}
    {%- set book_title = book.title if book.title else book.name -%}
    {%- set book_title = title if title else book_title -%}
    {% set srcset = book|get_cover_srcset %}
    <img
        srcset="{{ srcset }}"
        src="{{ url_for('web.get_cover', book_id=book.id, resolution=0, cache_bust=book|last_modified) }}"
        title="{{ book_title }}"
        alt="{{ book_title }}"
    />
{%- endmacro %}