The following example shows how you can use the PopUpManager class to launch an modal Image control.
<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2007/10/18/using-the-image-control-as-a-pop-up/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout
="vertical"
        verticalAlign
="middle"
        backgroundColor
="white">

    
<mx:Style>
        global {
            modalTransparencyBlur: 0;
            modalTransparency: 0.9;
            modalTransparencyColor: black;
            modalTransparencyDuration: 500;
        }
    
</mx:Style>

    
<mx:Script>
        
<![CDATA[
            import mx.controls.Image;
            import mx.managers.PopUpManager;

            private const IMG_PREFIX:String = "http://www.helpexamples.com/flash/images/";

            private function popIt(src:String):void {
                var img:Image = new Image();
                img.addEventListener(MouseEvent.CLICK, img_click);
                img.addEventListener(Event.COMPLETE, img_complete);
                img.load(IMG_PREFIX + src);
                img.toolTip = img.source.toString();
                PopUpManager.addPopUp(img, this, true);
            }

            private function img_complete(evt:Event):void {
                var img:Image = evt.currentTarget as Image;
                img.width = img.contentWidth;
                img.height = img.contentHeight;
                PopUpManager.centerPopUp(img);
            }

            private function img_click(evt:MouseEvent):void {
                var img:Image = evt.currentTarget as Image;
                PopUpManager.removePopUp(img);
            }
        
]]>
    
</mx:Script>

    
<mx:ApplicationControlBar dock="true">
        
<mx:Button label="image 1" click="popIt('image1.jpg');" />
        
<mx:Button label="image 2" click="popIt('image2.jpg');" />
        
<mx:Button label="image 3" click="popIt('image3.jpg');" />
    
</mx:ApplicationControlBar>

</mx:Application>