Set an image as a Button's content in style

I have a WPF button defined like this:

<Button Style="{StaticResource RevertButtonStyle}" />

Here is how the style looks:

<Style x:Key="RevertButtonStyle" TargetType="{x:Type Button}">
    <Setter Property="Height" Value="25" />
    <Setter Property="Width" Value="20" />
    <Setter Property="Margin" Value="3,0,0,0" />
</Style>

How do I change the style to specify the Content to use an image called "revert.png"?

Thanks.