If you want to add image in window background of WPF application then you should use Window.Background tag in xaml. Before set the image in background you must to add the image in the solution explorer. If you add image in the subfolder then specified the path of the folder like below example.
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:SplashSCreen"
mc:Ignorable="d"
Title="MainWindow" Height="500" Width="525">
<Window.Background>
<ImageBrush ImageSource="image/nokia.jpg" />
</Window.Background>
<Grid>
</Grid>
</Window>