Change the quality of your WordPress thumbnails

Add this to your theme’s functions.php:

function thumbnail_quality( $quality ) {
    return 100;

add_filter( 'jpeg_quality', 'thumbnail_quality' );
add_filter( 'wp_editor_set_quality', 'thumbnail_quality' );

Change 100 to anything you want. 95 would be the step in between default quality and max quality. it also works with images being resized via the new WP_Image_Editor class that was released with WordPress 3.5.