Customization login page - Coovachilli

We need to customize the captive portal login page of Coovachilli, we have to put an image as presentation and two buttons into the same image, this home screen should look good on laptop, iphone, ipad or other device mobil and buttons should work perfectly. One of the buttons will go to surf the Internet, and the other will redirect to an external url. For more details you may contact me. Please see picture for a better idea of ​​what is explained here.