// in functions.php 
<?php
function user_has_avatar() {
  global $bp;
  if ( !bp_core_fetch_avatar( array( 'item_id' => $bp->loggedin_user->id, 'no_grav' => true ) ) )
   return false;
   return true;
}
?>


// at the bottom of header.php 

<?php 
global $bp;
if( !bp_get_user_has_avatar()) :
?>
	<div class="alert">
	    <button class="close" data-dismiss="alert">×</button>
	    <strong>Warning !</strong> My message ....l.
	    </div>
<?php
endif;

// test two
if( !user_has_avatar() ) :
?>
	<div class="alert">
	    <button class="close" data-dismiss="alert">×</button>
	    <strong>Warning !</strong> My message ....l.
	    </div>
<?php
endif;

?>