move it to where? to other place or center the image on the same page??
if you want to center right or left the image in the same place, then you need to replace the code with something like this:
$document->add(new Comment("<div class=''><img src='{$this->avatar}' width='100'></div>"));
i played with the code and that works, i center mine with the code i have in CSS (<div class='center'>)
now if you want to display the avatar in another page... then you will need to call the database and other things to get the url of the user and display it in a $document->add(new Comment(""));
haven't tried to display the avatar in another page that's not the userprofile.