Image Fade In / Out With MouseEnter Event

I'm trying to make an image fade in/out when the user hovers over the button. The fadein works when I enter the button but not when I leave it. How can I make this work?

Animation Code:
Private Sub GreenBg_animation()
Dim myduration As Duration = New Duration(TimeSpan.FromMilliseconds(250))
Dim da As DoubleAnimation = New DoubleAnimation
da.Duration = myduration
Dim sb As Storyboard = New Storyboard
[code] .....

Button code:
Private Sub btn_2_MouseEnter(sender As Object, e As System.Windows.Input.MouseEventArgs) Handles btn_2.MouseEnter
GreenBg_animation()
End Sub