Central Theme Header User Icons

No problem! topic.posters.1.user is what you’re looking for (the first reply). If it exists, use that, else default to the OP.

This is how it’s currently structured in Central—

{{#if topic.posters.1.user}}
  <span class="tli__last-reply">
    {{d-icon "reply"}}
    <a href="{{topic.lastPoster.user.userPath}}" data-user-card="{{topic.lastPoster.user.username}}">
      {{~topic.lastPoster.user.username~}}
    </a>
    {{theme-i18n "topic_list_item.replied"}}
    <a href={{topic.lastPostUrl}}>
      {{format-date topic.bumpedAt format="medium" leaveAgo="true"}}
    </a>
  </span>
{{else}}
  <span class="tli__last-reply">
    {{d-icon "m-post_add"}}
    <a href="{{topic.posters.0.user.userPath}}" data-user-card="{{topic.posters.0.user.username}}">
      {{~topic.posters.0.user.username~}}
    </a>
    posted
    <a href={{topic.lastPostUrl}}>
      {{format-date topic.bumpedAt format="medium" leaveAgo="true"}}
    </a>
  </span>
{{/if}}