bulma/docs/_includes/elements/responsive-image.html

<img
  src="{{ site.url }}/images/{{ include.path }}.{{ include.extension }}"
  srcset="{{ site.url }}/images/{{ include.path }}.{{ include.extension }} 1x,
          {{ site.url }}/images/{{ include.path }}@2x.{{ include.extension }} 2x,
          {{ site.url }}/images/{{ include.path }}@3x.{{ include.extension }} 3x"
  alt="{{ include.alt }}"
  width="{{ include.width }}"
  height="{{ include.height }}">