Question

How to change image visibility from another Window`s button click event in WPF C#?

Answer 1

Any of the thousand possible ways. The simplest would be:

partial class Window2 : Window
{
    ...
    private Window1 _otherWindow;
    private void OnClick(object sender, RoutedEventArgs e)
    {
        _otherWindow.image.Visibility = Visibility.Collapsed;
    }
}

A better way would be to bind the image visibility to the ViewModel’s property, and change that property in the click handler.

Or you can associate a command with the button, and change the ViewModel of the window containing the image as a part of the business logic.

Etc. etc. etc.