Default logo is basic type of logo. Which will call in every header and every devices. So, first and foremost important logo format is default logo. But if you loss image quality in retina devices, you've to upload retina logos too.

Retina logo is only for retina devices. So, please upload 2x size from your default logo. You can also control this large logo width and height from our theme options. So, you'll not loss logo image quality in retina devices.

This is just a login page logo of your site admin. There is no more trick on it. If you not upload any logo in this area, you'll get default WordPress logo as it is. Login link will be like : http://yoursite.com/wp-login.php