Back ground in WPF application can be set using XAML at design time or at run time. Using XAML we do it as follows

 <Window.Background>
        <ImageBrush ImageSource="images\abstract-art-artistic-background-1103970 (1).jpg"/>
    </Window.Background>

At Runtime

Using some code you can change back ground of WPF /C# application using ImageBrush and BitmapImage object as follows

• Create Uri ,which defines the path of the image
• Using Uri define a bitmap image object which can source for ImageBrush
• Create a BitmapImage object and set the ImageBrush source
• Set Background property with ImageBrush

All the above can be done with inline code as follows

 ImageBrush imageBrush = new ImageBrush(new BitmapImage(new Uri("D:\images\abstract-art-artistic-background-1103970 (1).jpg",UriKind.RelativeOrAbsolute)));
                this.Background = imageBrush;