There are several plugins available that allow you to do some white label branding for Wordpress, you may want to look into some of those.
Alternatively you can do this: connect to the root of your WP install using your FTP program of choice. Navigate to the directory wp-admin/images. Upload (and replace the existing) image you want to use on the login screen – this image should be named logo-login.png and be sized at about 274 x 63.