Below is the style for your button and backgroung image is set to it.You can change the source of the ImageBrush to the one which you wanted .
<Style x:Key="ButtonStyle1" TargetType="{x:Type Button}">
<Setter Property="FocusVisualStyle" Value="{StaticResource ButtonFocusVisual}"/>
<Setter Property="Background" >
<Setter.Value>
<ImageBrush ImageSource="pic.png"></ImageBrush>
</Setter.Value>
</Setter>
<Setter Property="BorderBrush" Value="{StaticResource ButtonNormalBorder}"/>
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
<Setter Property="HorizontalContentAlignment" Value="Center"/>
<Setter Property="VerticalContentAlignment" Value="Center"/>
<Setter Property="Padding" Value="1"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Button}">
<Microsoft_Windows_Themes:ButtonChrome x:Name="Chrome" BorderBrush="{TemplateBinding BorderBrush}" Background="{TemplateBinding Background}" RenderMouseOver="{TemplateBinding IsMouseOver}" RenderPressed="{TemplateBinding IsPressed}" RenderDefaulted="{TemplateBinding IsDefaulted}" SnapsToDevicePixels="true">
<ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
</Microsoft_Windows_Themes:ButtonChrome>
<ControlTemplate.Triggers>
<Trigger Property="IsKeyboardFocused" Value="true">
<Setter Property="RenderDefaulted" TargetName="Chrome" Value="true"/>
</Trigger>
<Trigger Property="ToggleButton.IsChecked" Value="true">
<Setter Property="RenderPressed" TargetName="Chrome" Value="true"/>
</Trigger>
<Trigger Property="IsEnabled" Value="false">
<Setter Property="Foreground" Value="#ADADAD"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
And then just use it:
<StackPanel Orientation="Horizontal" Height="100px" VerticalAlignment="Top">
<StackPanel.Resources>
<Style TargetType="Button">
<Setter Property="Margin" Value="2,4" />
</Style>
</StackPanel.Resources>
<Button Width="127px" Height="79px" VerticalAlignment="Bottom" Style="{StaticResource ButtonStyle1}">
</Button>
<Button Style="{StaticResource ButtonStyle1}" >A</Button>
<Button Style="{StaticResource ButtonStyle1}">R</Button>
<Button Style="{StaticResource ButtonStyle1}">S</Button>
</StackPanel>