$size = getimagesize(Yii::$app->basePath.'/web/im.jpg');
        if($size[0] > 600 || $size[1] > 400)
        {
 /*
  * PHP GD
  * resize an image using GD library
  */

// File and new size
//the original image has 800x600
            $filename = Yii::$app->basePath.'/web/im.jpg';
//the resize will be a percent of the original size
            $percent = 0.2;

// Content type
            header('Content-Type: image/jpeg');

// Get new sizes
            list($width, $height) = getimagesize($filename);
            $newwidth = $width * $percent;
            $newheight = $height * $percent;

// Load
            $thumb = imagecreatetruecolor(100, 100);
            $source = imagecreatefromjpeg($filename);

// Resize
            imagecopyresized($thumb, $source, 0, 0, 0, 0, 100, 100, $width, $height);

// Output and free memory
//the resized image will be 400x300
            imagejpeg($thumb);
            imagedestroy($thumb);
        }