Use ImageBrush as the TextBox Background
<UserControl x:Class='SilverlightApplication3.MainPage'
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'
mc:Ignorable='d'
d:DesignWidth='640'
d:DesignHeight='480'>
<TextBox Text="Image TextBox"
Foreground="Black"
Height="200" Width="300">
<TextBox.Background>
<ImageBrush ImageSource="image.jpg"
Stretch="Fill"/>
</TextBox.Background>
</TextBox>
</UserControl>
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