<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2007/11/02/changing-a-button-controls-icon-when-the-button-is-disabled/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white" viewSourceURL="srcview/index.html">

    <mx:Style>
        .addButton {
            icon: Embed(source="images/bullet_add.png");
            disabledIcon: Embed(source="images/bullet_delete.png");
        }
    </mx:Style>

    <mx:ApplicationControlBar dock="true">
        <mx:CheckBox id="checkBox"
                label="enabled:"
                labelPlacement="left"
                selected="true" />
    </mx:ApplicationControlBar>

    <mx:Button label="Add"
        enabled="{checkBox.selected}"
        styleName="addButton" />

</mx:Application>