gridview command field image button delete not firing

Question

Hi, I'm implementing gridview crud operation. I used command field with button type button, all the functions (edit, update,delete) worked properly. But when I changed command field button type to image, delete function is not firing. When I click on Delete icon, Javascript alert is coming, and when I click ok, it is not firing OnRowDeleting function, Below is my code

<asp:GridView ID="grdCenterVisitDate" runat="server" AutoGenerateColumns="false" OnRowDataBound="OnRowDataBound"
DataKeyNames="CenterVisitID" OnRowEditing="OnRowEditing" OnRowCancelingEdit="OnRowCancelingEdit" PageSize = "5" AllowPaging ="true" OnPageIndexChanging = "OnPaging"
OnRowUpdating="OnRowUpdating" OnRowDeleting="OnRowDeleting" EmptyDataText="No records has been added."
>
<Columns>
<asp:TemplateField HeaderText="Center Visit date">
<ItemTemplate>
<asp:Label ID="lblName" runat="server" Text='<%# Eval("CenterVisitDate") %>'></asp:Label>
</ItemTemplate>
<EditItemTemplate>
<asp:TextBox ID="txtCenterVisitDate" runat="server" ValidationGroup="vgManageVisitDate" Text='<%# Eval("CenterVisitDate") %>'></asp:TextBox>
<asp:RequiredFieldValidator ID="rfvManageVisitDate" runat="server" ControlToValidate="txtCenterVisitDate" ErrorMessage="Date is required" Display="Dynamic" ValidationGroup="vgManageVisitDate"></asp:RequiredFieldValidator>
<ACT:CalendarExtender ID="cetxtStartDate" runat="server" TargetControlID="txtCenterVisitDate"></ACT:CalendarExtender>
</EditItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="CenterVisitType">
<ItemTemplate>
<asp:Label ID="lblCenterVisitType" runat="server" Text='<%# Eval("CenterVisitType") %>'></asp:Label>
</ItemTemplate>
<EditItemTemplate>
<asp:DropDownList runat="server" ID="ddlCenterVisit" SelectedValue='<%# Bind("CenterVisitTypeID") %>'>
<asp:ListItem Text="In-Person Class hours" Value="1"></asp:ListItem>
<asp:ListItem Text="In-Person Consultation" Value="2"></asp:ListItem>
<asp:ListItem Text="On-Line Consultation" Value="3"></asp:ListItem>
</asp:DropDownList>
</EditItemTemplate>
</asp:TemplateField>
<asp:CommandField ButtonType="Image" ShowEditButton="true" ShowCancelButton="true" ShowDeleteButton="true" ValidationGroup="vgManageVisitDate" EditImageUrl="~/Images/site/goals/icon-edit-sm.png" DeleteImageUrl="~/Images/site/goals/icon-delete-sm.png" UpdateImageUrl="~/Images/site/goals/icon-update-sm.png" CancelImageUrl="~/Images/site/goals/icon-cancel-sm.png" ControlStyle-CssClass="btn btn-cv-edit"/>
</Columns>
</asp:GridView>

protected void Page_Load(object sender, EventArgs e)
{
if (Request.QueryString["CenterID"] != null)
strCenterID = Request.QueryString["CenterID"];
if (!IsPostBack)
{
////txtDate1.StartDate = DateTime.Now;
fillCenterVisitDate();
}
else
dashboardID = 2;
}
private void fillCenterVisitDate()
{
DataSet ds = new DataSet();
ds = gdh.GetCenterVisitDateByDashboardID(2);
grdCenterVisitDate.DataSource = ds;
grdCenterVisitDate.DataBind();
}
#region CenterVisitDateGrid
protected void OnRowEditing(object sender, GridViewEditEventArgs e)
{
grdCenterVisitDate.EditIndex = e.NewEditIndex;
this.fillCenterVisitDate();
}
protected void OnRowUpdating(object sender, GridViewUpdateEventArgs e)
{
GridViewRow row = grdCenterVisitDate.Rows[e.RowIndex];
int gridId = Convert.ToInt32(grdCenterVisitDate.DataKeys[e.RowIndex].Values[0]);
DateTime centerVisitDate = Convert.ToDateTime((row.FindControl("txtCenterVisitDate") as TextBox).Text);
Int16 CenterVisit = Convert.ToInt16((row.FindControl("ddlCenterVisit") as DropDownList).SelectedValue);
int i = gdh.updateCenterVisitDate(gridId, CenterVisit, centerVisitDate, user.UserID);
grdCenterVisitDate.EditIndex = -1;
this.fillCenterVisitDate();
}
protected void OnRowCancelingEdit(object sender, EventArgs e)
{
grdCenterVisitDate.EditIndex = -1;
this.fillCenterVisitDate();
}
protected void OnRowDeleting(object sender, GridViewDeleteEventArgs e)
{
int centerVisitId = Convert.ToInt32(grdCenterVisitDate.DataKeys[e.RowIndex].Values[0]);
int i = gdh.DelCenterVisit(centerVisitId);
this.fillCenterVisitDate();
}
protected void OnRowDataBound(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.DataRow && e.Row.RowIndex != grdCenterVisitDate.EditIndex)
{
(e.Row.Cells[2].Controls[2] as LinkButton).Attributes["onclick"] = "return confirm('Do you want to delete this row?');";
}
}
protected void OnPaging(object sender, GridViewPageEventArgs e)
{
grdCenterVisitDate.PageIndex = e.NewPageIndex;
this.fillCenterVisitDate();
}
#endregion