How do you completely remove the button border in wpf?

Try this

<Button BorderThickness="0"      Style="{StaticResource {x:Static ToolBar.ButtonStyleKey}}" >...

You may have to change the button template, this will give you a button with no frame what so ever, but also without any press or disabled effect:

    <Style x:Key="TransparentStyle" TargetType="{x:Type Button}">        <Setter Property="Template">            <Setter.Value>                <ControlTemplate TargetType="Button">                    <Border Background="Transparent">                        <ContentPresenter/>                    </Border>                </ControlTemplate>            </Setter.Value>        </Setter>    </Style>

And the button:

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

What you have to do is something like this:

<Button Name="MyFlatImageButton"        Background="Transparent"        BorderBrush="Transparent"        BorderThickness="0"         Padding="-4">   <Image Source="MyImage.png"/></Button>

Hope this is what you were looking for.

Edit: Sorry, forgot to mention that if you want to see the button-border when you hover over the image, all you have to do is skip the Padding="-4".