When using LoginPress on the multisite setup you may require to change the default LoginPress theme background image. The following filter will help you achieve the task
function loginpress_default_bg_callback( $background ) {
$background = 'https://cdn.pixabay.com/photo/2019/07/19/09/40/flowered-balloon-flower-4348351_960_720.jpg';
return $background;
}
add_filter( 'loginpress_default_bg', 'loginpress_default_bg_callback' );
Here $background contains the URL for the background image. Make sure the image is optimized, otherwise it can cause the login page to load slow.