Make good thumbnails in WordPress

/*
* add post thumbnails 
*/
add_theme_support("post-thumbnails");
add_image_size("slider", 500, 350, true);
add_image_size("slidertop", 120, 80, true);
add_image_size("topweb", 60, 60, true);

add_filter("jpeg_quality", "custom_jpg_compression");
/*
* make 100 percent quality for jpeg
*/
function custom_jpg_compression($args) {
    return 100;
}