If you're using Enterprise Edition, you can change the logo displayed on the login screen, favicon, and game panel to match your company logo. To do this, navigate to the admin panel on your instance.
On the settings page, under the branding tab, you should see options to upload your logos - with some examples showing how the images will look on the page. To upload a new logo, click the image, and after it's uploaded, click Publish.
For further customisation, check out our Injecting CSS & JS guide.