You can do this a couple different ways. The Swap Image behavior is probably the easiest. Create two images with the same dimensions, one for your off state and one for the mouse-over state.
Place the Off State image on your page
In the Tag Inspector click the Behaviors button
Click the Add Behavior drop down button
Choose Swap Image
Select the image you just placed
Set Source to your Mouse over state
You could do it with CSS and actually have the div your image is in move when you mouse over it.
In your header:
Then in your <body>
<div id="buttondiv">your image or whatever here</div>
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