JPG compression

Hi,

I have a problem with JPG images compression. I’ve placed the following code in functions.php (in section for custom functions):

add_filter('jpeg_quality', function($arg){return 100;});

If I change the value to, for example, 50 – it’s working nad images look bad. But with value set to 100 images still look highly compressed. I can see that every time I’m uploading an image WordPress is compressing it and adding “_720x340” to it’s name.

What should I do to get it working?
Thanks in advance.