Another try :p
<?php
use yii\imagine\Image;
use Imagine\Image\Box;
use Imagine\Image\Point;
// ...
$thumbnail = Image::thumbnail($save_path, $img_size, $img_size);
$size = $thumbnail->getSize();
if ($size->getWidth() < $img_size or $size->getHeight() < $img_size) {
$white = Image::getImagine()->create(new Box($img_size, $img_size));
$thumbnail = $white->paste($thumbnail, new Point($img_size / 2 - $size->getWidth() / 2, $img_size / 2 - $size->getHeight() / 2));
}
$thumbnail->save($save_path);