WPF Storyboard issue

In the following template, the button is displayed with Normal image when the application is started, but when the mouse is over, the button doesn't get changed to m开发者_运维知识库ouse over image. Appreciate your help!!! I want some solution without major changes in the design.

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I found the problem.. I should be having the Opacity like below.. not as listed in the code above..

<Border x:Name="imgNormal" Opacity="0">