There's a couple ways to do this. You can either do a template conditional...
<if condition="$post[username] == "InsertUN">
<img src="http://link.to/image" />
<else />
$post[musername]
</if>
Or add a plug-in to do it. You would hook it to postbit_complete... the code should look like this...
if ($this->post["username"] == "Username") {
$this->post["musername"] = "<img src=\"http://link.to/image\">";
}
Obviously if you want more usernames to have it you would keep adding if statements. If you wanted to make it more involved you could add a custom profile field and just do something like...
if ($this->post["fieldXX"] != "") {
$this->post["musername"] = "<img src=\"". $this->post["musername"] ."\">";
}
Obviously replace "XX" with the number of the field you created. You put a URL in there and it will switch their username to that image.
I can't test all this at the moment, but everything should be correct.