Forms Data Controls :: Edit Grid Data In A Modal Popup

I have a GridView, and I want to enhance the edit mode, so, I was thinking in modal popup to edit the data when the user clicks the edit button. I used a templated filed with an image to edit data. In the GridView_RowDataBound code, I add an attribute to the image on each row bound...

Dim EditarActividad As ImageButton = e.Row.FindControl("EditarActividad")
EditarActividad.Attributes.Add("onclick", "launchModal")

I have a button TEST the pop-up modal frame, and it works fine:

<asp:Button ID="ModalPopupButton" runat="server" Text="Modal Popup" />
<asp:ModalPopupExtender ID="ModalPopupExtender" runat="server"
TargetControlID ="ModalPopupButton"
PopupControlID ="ModalPanel"
OkControlID ="OkButton"
DropShadow="true"
BackgroundCssClass="modalBackground"
RepositionMode="RepositionOnWindowResizeAndScroll" />

Also, I added the Javascritp "launchModal", on the client side to simulate the ModalPopupButton_Click...

<script type="text/javascript">
var launch = false;
function launchModal() {
launch = true;
}
function pageLoad() {
if (launch) {
var button = $find("ModalPopupButton");
button.click();
}
}
</script>

Why when I click the image on the GridView shows the modal popup frame on its original position (below the grid), and not floating on the center of the screen? (If I click the Button, it works fine).