AttachedProperties.cs

public static class AttachedProperties
{
    public static readonly DependencyProperty IconProperty = DependencyProperty.RegisterAttached(
        "Icon",
        typeof (StreamGeometry),
        typeof (AttachedProperties),
        new PropertyMetadata(default(StreamGeometry))
        );

    public static void SetIcon(UIElement element, StreamGeometry value)
    {
        element.SetValue(IconProperty, value);
    }
    public static StreamGeometry GetIcon(UIElement element)
    {
        return (StreamGeometry)element.GetValue(IconProperty);
    }
}

Style.xaml

<Window x:Class="ImagesPanelTemp.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
        xmlns:local="clr-namespace:ImagesPanelTemp"
        Title="MainWindow" SizeToContent="WidthAndHeight">
    <Window.Resources>
        <Style x:Key="ImageButton" TargetType="{x:Type Button}">
            <Setter Property="HorizontalContentAlignment" Value="Stretch" />
            <Setter Property="ContentTemplate">
                <Setter.Value>
                    <DataTemplate>
                        <Grid>
                            <Path Data="{Binding Path=(local:AttachedProperties.Icon), RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Button}}}"
    HorizontalAlignment="Left" Margin="8,0,0,0" Height="100" Width="100" Fill="Blue" Stretch="UniformToFill" Stroke="Red" StrokeThickness="6"/>
                            <TextBlock Text="{TemplateBinding Content}" HorizontalAlignment="Center" />
                        </Grid>
                    </DataTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Window.Resources>
    <Grid>
        <Button Content="Test" local:AttachedProperties.Icon="M1,0 1,1 0,1 z"  Style="{StaticResource ImageButton}"/>
    </Grid>

</Window>

Usage.Xaml

<Button Content="Pressing" local:AttachedProperties.Icon="{StaticResource Picka}" Style="{StaticResource ImageButton}"/>