Are you sure you want to use an image? If the image is just a solid color, you're probably better off just setting the button's background to white and adjusting its opacity. Like:
<Button Background="White" Opacity="0.5" />
Where Opacity can be any value between 0 and 1.
EDIT: I'm not sure what you mean by "shades." Are you aware that you can set the background of a Button to a gradient? You'll want to change the colors I'm sure, but it will look something like this:
<Button>
<Button.Background>
<LinearGradientBrush>
<GradientStop Color="Yellow" Offset="0.0" />
<GradientStop Color="Orange" Offset="0.5" />
<GradientStop Color="Red" Offset="1.0" />
</LinearGradientBrush>
<Button.Background>
</Button>