<Button Background="{StaticResource PhoneContrastBackgroundBrush}" >
<Button.OpacityMask>
<ImageBrush ImageSource="/Images/icon.png"/>
</Button.OpacityMask>
</Button>
icon.png должен быть белого цвета
[РЕДАКТИРОВАТЬ]
ИЛИ (обведите их вокруг) с использованием изображения SDK (appbar.basecircle.rest.png)
<Style x:Key="FlatStyle" TargetType="Button">
<Setter Property="Background" Value="Transparent"/>
<Setter Property="BorderBrush" Value="{StaticResource PhoneForegroundBrush}"/>
<Setter Property="Foreground" Value="{StaticResource PhoneForegroundBrush}"/>
<Setter Property="BorderThickness" Value="{StaticResource PhoneBorderThickness}"/>
<Setter Property="FontFamily" Value="{StaticResource PhoneFontFamilySemiBold}"/>
<Setter Property="FontSize" Value="{StaticResource PhoneFontSizeMediumLarge}"/>
<Setter Property="Padding" Value="10,3,10,5"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="Button">
<Grid Background="Transparent">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="MouseOver"/>
<VisualState x:Name="Pressed">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentContainer">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneBackgroundBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="ButtonBackground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneForegroundBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="ButtonBackground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneForegroundBrush}"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Foreground" Storyboard.TargetName="ContentContainer">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneDisabledBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="BorderBrush" Storyboard.TargetName="ButtonBackground">
<DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PhoneDisabledBrush}"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Background" Storyboard.TargetName="ButtonBackground">
<DiscreteObjectKeyFrame KeyTime="0" Value="Transparent"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Border x:Name="ButtonBackground" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" CornerRadius="0" Margin="{StaticResource PhoneTouchTargetOverhang}">
<Grid>
<Canvas Width="{TemplateBinding Width}" Height="{TemplateBinding Height}">
<Rectangle Width="{TemplateBinding Width}" Height="{TemplateBinding Height}" Fill="{StaticResource PhoneForegroundBrush}">
<Rectangle.OpacityMask>
<ImageBrush ImageSource="/appbar.basecircle.rest.png"/>
</Rectangle.OpacityMask>
</Rectangle>
<Rectangle Width="{TemplateBinding Width}" Height="{TemplateBinding Height}" Fill="{StaticResource PhoneForegroundBrush}" OpacityMask="{TemplateBinding BorderBrush}"/>
</Canvas>
<ContentControl x:Name="ContentContainer" ContentTemplate="{TemplateBinding ContentTemplate}" Content="{TemplateBinding Content}" Foreground="{TemplateBinding Foreground}" HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" Padding="{TemplateBinding Padding}" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"/>
</Grid>
</Border>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Использование:
<Button Style="{StaticResource FlatStyle}" BorderThickness="0" Width="48" Height="48">
<Button.BorderBrush>
<ImageBrush ImageSource="/appbar.favs.rest.png"/>
</Button.BorderBrush>
</Button>