create resource in the App.xaml
<ImageSource x:Key="test1">banner_img_1.jpg</ImageSource>
<ImageSource x:Key="test2">banner_img_2.jpg</ImageSource>
Now change the source like that
private void image1_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
image1.Source = this.FindResource("test2") as ImageSource;
}