Problem with Image Manipulation Library

I want to compress the image by 60%. My code is given bellow. I have uploaded an image of 627Kb. The code block is executed but not compressing. Is there any error in my code?

Thanks in advance.

PHP Code:

$this->load->library('image_lib');
if($this->upload->data('file_size') > 300){
    $config['image_library']    = 'gd2';
    $config['source_image']     = $this->upload->data('full_path');
    $config['quality']          = '60%'; // Also tried $config['quality']          = 60;
    $config['new_image']        = $this->upload->data('full_path');
    $this->image_lib->clear();
    $this->image_lib->initialize($config);
    $this->image_lib->resize();
    echo 'I am ok';
} 


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