Something like this should work (you may have to tweak it a little to suite your needs):
<ToggleButton Margin="110,100,247,179">
<ToggleButton.Style>
<Style TargetType="{x:Type ToggleButton}">
<Setter Property="Content">
<Setter.Value>
<Image Source="/WpfApplication1;component/off.png" />
</Setter.Value>
</Setter>
<Style.Triggers>
<Trigger Property="IsChecked" Value="True">
<Setter Property="Content">
<Setter.Value>
<Image Source="/WpfApplication1;component/on.png" />
</Setter.Value>
</Setter>
</Trigger>
</Style.Triggers>
</Style>
</ToggleButton.Style>
</ToggleButton>