You swapped your x and y values. Assuming your image is in a panel the size of the window and has VerticalAlignment="Top"
and HorizontalAlignment="Left"
you could do this:
var bounds = Window.Current.Bounds;
var r = new Random();
int x = r.Next(0, (int)bounds.Right - (int)image.ActualWidth);
int y = r.Next(140, (int)bounds.Bottom - (int)image.ActualHeight);
image.Margin = new Thickness(x, y, -x, -y);