Full code after the jump.

View MXML

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2007/11/04/detecting-whether-an-image-loaded-successfully-in-flex/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout="vertical"
        verticalAlign="middle"
        backgroundColor="white">

    <mx:Script>
        <![CDATA[
            import mx.controls.Alert;
            import flash.events.HTTPStatusEvent;

            private function img_httpStatus(evt:HTTPStatusEvent):void {
                switch (evt.status) {
                    case 200:
                        // Valid image, do nothing.
                        break;
                    case 404:
                        // Invalid image, PANIC!
                        Alert.show("invalid image!", evt.status.toString());
                        break;
                    default:
                        Alert.show(evt.toString(), evt.status.toString());
                        break;
                }
            }

            private function button_click(evt:MouseEvent):void {
                img.source = textInput.text;
            }
        ]]>
    </mx:Script>

    <mx:ApplicationControlBar dock="true">
        <mx:HDividedBox width="100%">
            <mx:TextInput id="textInput"
                    text="http://www.helpexamples.com/flash/images/image1.jpg" />
            <mx:Button label="load image"
                    width="100"
                    minWidth="100"
                    maxWidth="100"
                    click="button_click(event);" />
            <mx:Spacer width="100%" />
        </mx:HDividedBox>
    </mx:ApplicationControlBar>

    <mx:Image id="img"
            httpStatus="img_httpStatus(event);" />

</mx:Application>

View source is enabled in the following example.