XF 2.0 Set Default Icons For Threads

[String Theory]

Hello!

I'd like to use image icons for all threads instead of avatars. What I need is to set a default icon for all threads in my forum. Actually I found the solution for this one by putting this code

<p><img styles="mystyle/iconthread.png"></p>

in thread_macros instead of this

<xf:avatar user="$thread.User" size="s" defaultname="{$thread.username}" />
                <xf:if is="$thread.getUserPostCount()">
                    <xf:avatar user="$xf.visitor" size="s"
                        href=""
                        class="avatar--separated structItem-secondaryIcon"
                        title="{{ phrase('you_have_posted_x_messages_in_this_thread', {'count': $thread.getUserPostCount() }) }}" />
                </xf:if>

and using some css in extra.less, and that works fine.
Now I need to set another default icon for those threads which visitor has posted in. How can I do that? Please, help me with the code.

Thanks for any help