Fill image after postback problem

I have the following problem:

When I click the button, I want to fill in the image source and show the image inside the ModalPopupExtender module. To open ModalPopupExtender, I call __doPostBackthat cause UpdatePanel to do a postback and show the corresponding view with the image inside it.

 <script src="jquery-1.5.2.min.js" type="text/javascript"></script>
<script type="text/javascript">
    window.pageLoad = function () {
        $addHandler($get('btn1'), 'click', function (e) {
            __doPostBack('pbcTest', '');
            // setTimeout("PopulateImage()", 100);
            PopulateImage();
            $find('<%=mpePopup.ClientID%>').show();
            e.preventDefault();
        });
    }

    function PopulateImage() {
        $(".testImage").attr("src", "6.jpg");
    }

</script>

<ajaxtoolkit:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server" CombineScripts="false">
    </ajaxtoolkit:ToolkitScriptManager>
    <button id="btn1"> Click</button>
    <asp:LinkButton ID="lbTest" runat="server" Text="show popup" Style="display: none;" />
    <ajaxtoolkit:ModalPopupExtender runat="server" ID="mpePopup" TargetControlID="lbTest"
        PopupControlID="pnlPopup" BackgroundCssClass="modalBackground" CancelControlID="lbClose" />
    <asp:Panel runat="server" ID="pnlPopup" CssClass="modalPopup">
        <asp:LinkButton runat="server" ID="lbClose">Close</asp:LinkButton>
        <asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
            <ContentTemplate>
                <asp:MultiView runat="server" ID="mvPopup">
                    <asp:View ID="View1" runat="server">
                        <img class="testImage" />
                    </asp:View>
                </asp:MultiView>
                <cs:PostBackControl runat="server" ID="pbcTest" OnCallBack="pbcTest_CallBack" />
            </ContentTemplate>
        </asp:UpdatePanel>
    </asp:Panel>

 protected void pbcTest_CallBack(object sender, CallBackEventArgs e)
    {
        mvPopup.ActiveViewIndex = 0; 
    }

If after __doPostBackI call immediately PopulateImage(), the image is empty (although it is called PopulateImage()and the image appears for a while, but after that disappears, it can be seen in firebug).

But if I set a timeout setTimeout("PopulateImage()", 100);, an image will appear.

What could be causing this behavior?