[brightwash]

define ( ‘BP_AVATAR_DEFAULT_THUMB’, $img_url );

This was not working for me either, in the end this combination did. Turning off gravatar (first filter), and then a default bp image filter.

add_filter( 'bp_core_fetch_avatar_no_grav', '__return_true' );
add_filter( 'bp_core_default_avatar_user', 'override_default_avatar', 10, 2 );
function override_default_avatar( $default, $params ) {
	if ( !empty($params) ) {
		if ( 'thumb' == $params['type'] ) {
			return  get_stylesheet_directory_uri() . '/img/avatar-thumb.jpg'; 
		} 
	}
	return  get_stylesheet_directory_uri() . '/img/avatar.jpg'; 
}