Use a StackPanel to place an Icon at the top and text at the bottom.
<Button>
<stackpanel>
Orientation="Vertical">
<Image
Height="32"
Margin="4,4,4,4"
Source="Icons/MyButton.ico" />
<textblock>
Text="MyText"
Margin="4,4,4,4"
VerticalAlignment="Center" />
</textblock></stackpanel>
</Button>
(the formatting gets changed by the website but you get the idea)