<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2008/01/05/changing-a-vbox-containers-background-size-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white" viewSourceURL="srcview/index.html">

    <mx:Style>
        VBox {
            backgroundImage: Embed("Fx.png");
            backgroundColor: red;
        }
    </mx:Style>

    <mx:ApplicationControlBar dock="true">
        <mx:Form styleName="plain">
            <mx:FormItem label="percentWidth:">
                <mx:HSlider id="sliderW"
                        minimum="0"
                        maximum="100"
                        value="100"
                        liveDragging="true"
                        snapInterval="1"
                        tickInterval="10"
                        dataTipPrecision="0" />
            </mx:FormItem>
            <mx:FormItem label="percentHeight:">
                <mx:HSlider id="sliderH"
                        minimum="0"
                        maximum="100"
                        value="100"
                        liveDragging="true"
                        snapInterval="1"
                        tickInterval="10"
                        dataTipPrecision="0" />
            </mx:FormItem>
            <mx:FormItem label="backgroundSize:">
                <mx:ComboBox id="comboBox">
                    <mx:dataProvider>
                        <mx:Array>
                            <mx:Object label="auto" />
                            <mx:Object label="100%" />
                            <mx:Object label="90%" />
                            <mx:Object label="80%" />
                            <mx:Object label="70%" />
                            <mx:Object label="60%" />
                            <mx:Object label="50%" />
                            <mx:Object label="40%" />
                            <mx:Object label="30%" />
                            <mx:Object label="20%" />
                            <mx:Object label="10%" />
                            <mx:Object label="0%" />
                        </mx:Array>
                    </mx:dataProvider>
                </mx:ComboBox>
            </mx:FormItem>
        </mx:Form>
    </mx:ApplicationControlBar>

    <mx:HBox percentWidth="{sliderW.value}"
            percentHeight="{sliderH.value}">
        <mx:VBox backgroundSize="{comboBox.selectedItem.label}"
                width="100%"
                height="100%" />
    </mx:HBox>

</mx:Application>