您可以使用
LinearGradientBrush
<Image Source="image.png" Width="200" Height="400">
<Image.OpacityMask>
<LinearGradientBrush StartPoint="0.5,0.5" EndPoint="0.5,1">
<GradientStop Color="#FFFFFFFF" Offset="0"/>
<GradientStop Color="#00FFFFFF" Offset="1"/>
</LinearGradientBrush>
</Image.OpacityMask>
</Image>