<img> not working in RichText

Here is a very simple application to put this point across:

<?xml version="1.0" encoding="utf-8"?>
<s:Application
    xmlns:fx="http://ns.adobe.com/mxml/2009"
    xmlns:s="library://ns.adobe.com/flex/spark"
    xmlns:mx="library://ns.adobe.com/flex/mx"
    width="1000"
    creationComplete="application1_initializeHandler(event)"
    >
    <s:RichText
        id="richTxt"
        width="100%"
        textFlow="{textFlow}"
        horizontalCenter="0"
        verticalCenter="0"
        />

    <fx:Declarations>
    </fx:Declarations>
   
    <fx:Script>
        <![CDATA[
            import flashx.textLayout.elements.TextFlow;
            import spark.utils.TextFlowUtil;
           
            import mx.events.FlexEvent;
           
            import org.osmf.layout.AbsoluteLayoutFacet;
           
            [Bindable]
            private var textFlow:TextFlow;
           
            private var inputString:String =
                "<TextFlow whiteSpaceCollapse='preserve' xmlns='http://ns.adobe.com/textLayout/2008'>" +
                "<p>" +
                "<span fontFamily='Verdana' fontSize='18'>There should be an image between this...</span>" +
                "<img height='auto' width='auto' source='assets/images/math.png'/>" +
                "<span fontFamily='Verdana' fontSize='18'>... and this.</span>" +
                "<span></span>" +
                "</p>" +
                "</TextFlow>";
           
            protected function application1_initializeHandler(event:FlexEvent):void {
                textFlow = TextFlowUtil.importFromXML(new XML(inputString));
            }
           
        ]]>
    </fx:Script>
</s:Application>

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Here is math.png that you can save in assets/images/