You should set the style for the button instead,
and set the content property in the trigger
<Style TargetType="{x:Type Button}">
<Style.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Content" >
<Setter.Value>
<Image Source="/VeriPark.CTI.Demo;component/Images/answer-hover.PNG" Stretch="UniformToFill"/>
</Setter.Value>
</Setter>
</Trigger>
</Style.Triggers>
</Style>
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If that works for you, please mark it as an answer.