You can change the ZIndex in code.
Assuming this xaml:
<Canvas Grid.Row="1" Tap="gridTapped">
<Image x:Name="ImgA" Source="Assets/A.png" />
<Image x:Name="ImgB" Source="Assets/B.png" />
<Image x:Name="ImgC" Source="Assets/C.png" />
</Canvas>
You could change which image is on top with the following:
private int TopZindex = 10;
private void gridTapped(object sender, GestureEventArgs e)
{
var rand = new Random();
switch (rand.Next(0, 3))
{
case 0:
Canvas.SetZIndex(this.ImgA, ++TopZindex);
break;
case 1:
Canvas.SetZIndex(this.ImgB, ++TopZindex);
break;
case 2:
Canvas.SetZIndex(this.ImgC, ++TopZindex);
break;
}
}