By default, WordPress compress uploaded jpg images. This can be a good thing because it makes the file size smaller (not the dimension), but in some cases, image quality can be more important than filesize. If you need to prevent WordPress from compressing your images or you wish to specify what level of compression WordPress should apply, simply add this snippet.

Add this code to your functions.php file.

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