What does your PNG look like? If it is rectangular try this...
<mx:Button id="mybttn"
label="Click Me"
overSkin="@Embed(source='PathToPNG/filename1.png') "
upSkin="@Embed(source='PathToPNG/filename2.png')"
downSkin="@Embed(source='PathToPNG/filename3.png') "/>

If the PNG is a strange shape...like the shape of Texas...
There is a little more to it. Let me know and I can walk you through the other
way.