C#更换groupBox的背景图片

C#选择radioBox1和radioBox2更换groupBox2的背景图片，明明在资源项目文件里有这个图片，
但是一直提示
错误 CS1061 “object”未包含“Ub”的定义，并且找不到可接受第一个“object”类型参数的可访问扩展方法“Ub”(是否缺少 using 指令或程序集引用?)

<RadioButton Name="RadioBox1" Content="Option 1" Checked="RadioBox_Checked"/>
<RadioButton Name="RadioBox2" Content="Option 2" Checked="RadioBox_Checked"/>

<GroupBox Name="GroupBox2" Background="White">
    <GroupBox.Background>
        <ImageBrush ImageSource="Ua.png"/>
    </GroupBox.Background>
</GroupBox>


private void RadioBox_Checked(object sender, RoutedEventArgs e)
{
    if (RadioBox1.IsChecked == true)
    {
        GroupBox2.Background = new ImageBrush(new BitmapImage(new Uri("Ua.png", UriKind.Relative)));
    }
    else if (RadioBox2.IsChecked == true)
    {
        GroupBox2.Background = new ImageBrush(new BitmapImage(new Uri("Ub.png", UriKind.Relative)));
    }
}