WordPress automatically compresses images to 90 per cent of the original. While this isn’t such a big deal for most site owners, some people, like photographers, miss this extra 10 per cent.
To ensure the images on your site are at 100 per cent quality, add this to your theme’s functions.php file:

 

add_filter( 'jpg_quality', 'ns_high_jpg_quality' );
function ns_high_jpg_quality() {
  return 100;
}