You are dealing with a class. Most everything has to be inside a class method, not counting class variables/constants.
Second, you the syntax is slightly different when adding a class method, rather than a function, to a hook.
function widget( $args, $instance ) {
add_filter('get_avatar', array($this,'remove_photo_class'));
// the rest of your code
}
I would also edit your callback to remove itself:
function remove_photo_class($avatar) {
remove_filter('get_avatar', array($this,'remove_photo_class'));
return str_replace(' avatar', $image_align , $avatar);
}
Or explicitly remove it at the end of the widget
method:
function widget( $args, $instance ) {
add_filter('get_avatar', array($this,'remove_photo_class'));
// the rest of your code
remove_filter('get_avatar', array($this,'remove_photo_class'));
}
Or it might mess up things in other parts of the theme.