trying to filter the string in my functions.php file brought up the function already declared in wordpress includes, so i used a work-around.
In my case all I just wanted was to re-style the avatar, as I was using the same call get_avatar()
in my site header menu and in-page(two calls on the same page). each call required different styles to them.
To solve this I styled the first call using the already existing css class photo(you can also use the avatar, avatar-default or avatar-96 classes).
i used javascript to replace the class, width and height property of the second call see code below.
<div id="text-center" class="text-center">
<?php echo get_avatar( $current_user->ID ); ?>
</div>
<script>
document.getElementById("text-center").getElementsByTagName('img')[0].className = "userdp";
document.getElementById("text-center").getElementsByTagName('img')[0].width = "130";
document.getElementById("text-center").getElementsByTagName('img')[0].height = "80";
</script>