The Centova Cast logos are just PNG files in the theme/images/ subdirectory of your Centova Cast web interface directory.

To change the logo displayed in the user/admin area, replace logo.png with your own logo.  (You can use header-bg.png in the same directory as a background image if you want to retain the existing header background.)

To change the logo displayed on the login page, replace login-logo.png with your own logo.