В скине откройте тег script и сделайте:
[Embed(source="assets/images/button-up.png", scaleGridLeft = "4", scaleGridRight = "5", scaleGridTop = "2", scaleGridBottom = "3")]
[Bindable]
public var upImg:Class;
[Embed(source="assets/images/button-over.png", scaleGridLeft = "4", scaleGridRight = "5", scaleGridTop = "4", scaleGridBottom = "5")]
[Bindable]
public var overImg:Class;
[Embed(source="assets/images/button-disabled-skin.png", scaleGridLeft = "4", scaleGridRight = "5", scaleGridTop = "2", scaleGridBottom = "3")]
[Bindable]
public var disabledImg:Class;
[Embed(source="assets/images/button-over.png", scaleGridLeft = "4", scaleGridRight = "5", scaleGridTop = "4", scaleGridBottom = "5")]
[Bindable]
public var downImg:Class;
удалить все заливки по умолчанию и вместо этого использовать:
<s:BitmapImage source="{upImg}"
source.over="{overImg}"
source.down="{downImg}"
source.disabled="{disabledImg}"
width="100%"/>
И вуаля '