Full code after the jump.

View MXML

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2007/12/07/using-the-imagesnapshot-class-to-capture-images-as-jpegs-or-pngs-in-flex-3/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white">

    <mx:Script>
        <![CDATA[
            import mx.graphics.ImageSnapshot;
            import mx.graphics.codec.*;

            private const jpgEnc:JPEGEncoder = new JPEGEncoder();
            private const pngEnc:PNGEncoder = new PNGEncoder();

            private function captureImg(imgEnc:IImageEncoder):void {
                var ohSnap:ImageSnapshot;
                ohSnap = ImageSnapshot.captureImage(img, 0, imgEnc);
                textArea.text = ImageSnapshot.encodeImageAsBase64(ohSnap);
            }
        ]]>
    </mx:Script>

    <mx:ApplicationControlBar dock="true">
        <mx:ToggleButtonBar id="toggleButtonBar"
                itemClick="captureImg(event.item.data);">
            <mx:dataProvider>
                <mx:Array>
                    <mx:Object label="Capture as JPEG" data="{jpgEnc}" />
                    <mx:Object label="Capture as PNG" data="{pngEnc}" />
                </mx:Array>
            </mx:dataProvider>
        </mx:ToggleButtonBar>
    </mx:ApplicationControlBar>

    <mx:Image id="img"
            source="@Embed('images/flex_logo.jpg')" />

    <mx:TextArea id="textArea"
            editable="false"
            width="320"
            height="160" />

</mx:Application>

View source is enabled in the following example.