Your site under maintenance mode, so I can’t check it.
What exactly banner did you try to upload?
500 Internal server error means that there is some Fatal error in PHP script or on the server side. You can turn on debug mode in WordPress to see the error https://codex.wordpress.org/Debugging_in_WordPress
If it will not show any errors then contact your hosting providers and ask them to give you a server error log to check what is wrong.