WordPress plugin repository has a new login logo plugin that lets you use a custom login logo without any need to copy-paste logo path or manually upload logo image via FTP. You can easily select a custom login logo with a custom height directly from your admin area dashboard.
Installation & Usage: First install and activate Custom Login Logo plugin. After activation visit your admin area Dashboard -> Settings -> Custom Login Logo plugin settings page and upload your logo image simply by using in-built media manager, set a custom height, save settings and you are done. Log out ad check your login page to see how it works.