I used part of @dax theme component to show the author avatar : Discourse Topic Author
Here are the parts to add in the Header section
<td class='author topic-list-data'>
<a href="{{topic.creator.path}}" data-user-card="{{topic.creator.username}}">
{{#if (theme-setting 'show_author_username')}}
{{topic.creator.username}}
{{else}}
{{avatar topic.creator imageSize="45"}}
{{/if}}
</a>
</td>
inside the <script type='text/x-handlebars' data-template-name='list/topic-list-item.raw'> (...)</script>
And
{{raw "topic-list-header-column" number='true'}}
inside <script type='text/x-handlebars' data-template-name='topic-list-header.raw'> (...)</script>
Here is the complete code you can use
<script type='text/x-handlebars' data-template-name='list/topic-list-item.raw'>
{{~raw-plugin-outlet name="topic-list-before-columns"}}
{{#if bulkSelectEnabled}}
<td class="bulk-select">
<input type="checkbox" class="bulk-select">
</td>
{{/if}}
<td class='author topic-list-data'>
<a href="{{topic.creator.path}}" data-user-card="{{topic.creator.username}}">
{{#if (theme-setting 'show_author_username')}}
{{topic.creator.username}}
{{else}}
{{avatar topic.creator imageSize="45"}}
{{/if}}
</a>
</td>
<td class='main-link clearfix topic-list-data' colspan="1" role="rowheader">
{{~raw-plugin-outlet name="topic-list-before-link"}}
<span class='link-top-line'>
{{~raw-plugin-outlet name="topic-list-before-status"}}
{{~raw "topic-status" topic=topic}}
{{~topic-link topic class="raw-link raw-topic-link"}}
{{~#if topic.featured_link}}
{{~topic-featured-link topic}}
{{~/if}}
{{~raw-plugin-outlet name="topic-list-after-title"}}
{{~raw "list/unread-indicator" includeUnreadIndicator=includeUnreadIndicator
topicId=topic.id
unreadClass=unreadClass~}}
{{~#if showTopicPostBadges}}
{{~raw "topic-post-badges" unread=topic.unread newPosts=topic.displayNewPosts unseen=topic.unseen url=topic.lastUnreadUrl newDotText=newDotText}}
{{~/if}}
</span>
<div class="link-bottom-line">
{{#unless hideCategory}}
{{#unless topic.isPinnedUncategorized}}
{{category-link topic.category}}
{{/unless}}
{{/unless}}
<span class='topic-creator'>{{d-icon "user"}} <a href="/users/{{topic.creator.username}}" data-auto-route="true" data-user-card="{{topic.creator.username}}">{{topic.creator.username}}</a></span>
{{discourse-tags topic mode="list" tagsForUser=tagsForUser}}
{{raw "list/action-list" topic=topic postNumbers=topic.liked_post_numbers className="likes" icon="heart"}}
</div>
{{#if expandPinned}}
{{raw "list/topic-excerpt" topic=topic}}
{{/if}}
</td>
{{raw "list/posts-count-column" topic=topic}}
{{#if showLikes}}
<td class="num likes topic-list-data">
{{#if hasLikes}}
<a href='{{topic.summaryUrl}}'>
{{number topic.like_count}} {{d-icon "heart"}}</td>
</a>
{{/if}}
{{/if}}
<td class="num views {{topic.viewsHeat}} topic-list-data">{{number topic.views numberKey="views_long"}}</td>
<td class="last-post">
<div class='poster-avatar'>
<a href="{{topic.lastPostUrl}}" data-user-card="{{topic.last_poster_username}}">{{avatar topic.lastPosterUser imageSize="medium"}}</a>
</div>
<div class='poster-info'>
<span class='editor'><a href="/users/{{topic.last_poster_username}}" data-auto-route="true" data-user-card="{{topic.last_poster_username}}">{{topic.last_poster_username}}</a></span>
<br />
<a href="{{topic.lastPostUrl}}">
{{format-date topic.bumpedAt format="tiny"}}
</a>
</div>
</td>
</script>
<script type='text/x-handlebars' data-template-name='topic-list-header.raw'>
{{~raw-plugin-outlet name="topic-list-header-before"~}}
{{#if bulkSelectEnabled}}
<th class="bulk-select topic-list-data">
{{#if canBulkSelect}}
{{raw "flat-button" class="bulk-select" icon="list" title="topics.bulk.toggle"}}
{{/if}}
</th>
{{/if}}
{{raw "topic-list-header-column" order='default' name=listTitle bulkSelectEnabled=bulkSelectEnabled showBulkToggle=toggleInTitle canBulkSelect=canBulkSelect}}
{{raw "topic-list-header-column" sortable=sortable number='true' order='posts' name='replies'}}
{{#if showLikes}}
{{raw "topic-list-header-column" sortable=sortable number='true' order='likes' name='likes'}}
{{/if}}
{{#if showOpLikes}}
{{raw "topic-list-header-column" sortable=sortable number='true' order='op_likes' name='likes'}}
{{/if}}
{{raw "topic-list-header-column" sortable=sortable number='true' order='views' name='views'}}
{{raw "topic-list-header-column" sortable=sortable number='true' order='activity' name='activity'}}
</script>
<script type='text/x-handlebars' data-template-name='topic-list-header.raw'>
{{#if bulkSelectEnabled}}
<th class="bulk-select topic-list-data">
{{#if canBulkSelect}}
{{raw "flat-button" class="bulk-select" icon="list" title="topics.bulk.toggle"}}
{{/if}}
</th>
{{/if}}
{{raw "topic-list-header-column" number='true'}}
{{raw "topic-list-header-column" order='default' name=listTitle bulkSelectEnabled=bulkSelectEnabled showBulkToggle=toggleInTitle canBulkSelect=canBulkSelect}}
{{raw "topic-list-header-column" sortable=sortable number='true' order='posts' name='replies'}}
{{#if showLikes}}
{{raw "topic-list-header-column" sortable=sortable number='true' order='likes' name='likes'}}
{{/if}}
{{#if showOpLikes}}
{{raw "topic-list-header-column" sortable=sortable number='true' order='op_likes' name='likes'}}
{{/if}}
{{raw "topic-list-header-column" sortable=sortable number='true' order='views' name='views'}}
{{raw "topic-list-header-column" sortable=sortable number='true' order='activity' name='activity'}}
{{~raw-plugin-outlet name="topic-list-header-after"~}}
</script>
<script type="script/discourse-plugin">
(function(){
var TopicListItemView = require('discourse/components/topic-list-item').default;
TopicListItemView.reopen({
showCategory: function(){
return !this.get('controller.hideCategory') &&
this.get('topic.creator') &&
this.get('topic.category.name') !== 'uncategorized';
}.property()
});
})();
</script>
