Install the plugin, tell it where is your logo, and it will be in your login page. Based in Logo Login by Mark Jaquith.
This plugin is intended to allow you to change your login logo easily. You just need to upload it to any place and tell the system where it is. It differs from other plugins in its options and weight. It is intended to be lite and only change the logo.
Try to adjust your image to 310x51px.