Input String Not In Correct Format

Hi
I have just started using Visual Web Developer to produce
asp.net page. I am having a problem when inserting a new record into a
particular table, within the insertion form I have included a drop down
list which gathers informaton from another table. When I view the page
through the browser when I click the insert link I get an error message
on stating 'Input string not in correct format' . The code for the page is below. Does anyone know what
the problem could be?
 
Thanks
 
<%@ Page Language="VB" MasterPageFile="~/MasterPage.master" AutoEventWireup="false" CodeFile="Default.aspx.vb" Inherits="PhotoAdmin_Default" title="Untitled Page" %>
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
    <asp:Label ID="UserIdValue" runat="server"></asp:Label><br />
    <asp:SqlDataSource ID="pictureDataSource" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionString %>"
        DeleteCommand="DELETE FROM [Pictures] WHERE [PictureID] = @PictureID" InsertCommand="INSERT INTO [Pictures] ([UserId], [CategoryID], [Title], [Description], [UploadedOn]) VALUES (@UserId, @CategoryID, @Title, @Description, @UploadedOn)"
        SelectCommand="SELECT * FROM [Pictures] WHERE ([UserId] = @UserId) ORDER BY [UploadedOn] DESC"
        UpdateCommand="UPDATE [Pictures] SET [UserId] = @UserId, [CategoryID] = @CategoryID, [Title] = @Title, [Description] = @Description, [UploadedOn] = @UploadedOn WHERE [PictureID] = @PictureID">
        <DeleteParameters>
            <asp:Parameter Name="PictureID" Type="Int32" />
        </DeleteParameters>
        <UpdateParameters>
            <asp:Parameter Name="UserId" />
            <asp:Parameter Name="CategoryID" Type="Int32" />
            <asp:Parameter Name="Title" Type="String" />
            <asp:Parameter Name="Description" Type="String" />
            <asp:Parameter Name="UploadedOn" Type="DateTime" />
            <asp:Parameter Name="PictureID" Type="Int32" />
        </UpdateParameters>
        <SelectParameters>
            <asp:ControlParameter ControlID="UserIdValue" Name="UserId" PropertyName="Text" />
        </SelectParameters>
        <InsertParameters>
            <asp:Parameter Name="UserId" />
            <asp:Parameter Name="CategoryID" Type="Int32" />
            <asp:Parameter Name="Title" Type="String" />
            <asp:Parameter Name="Description" Type="String" />
            <asp:Parameter Name="UploadedOn" Type="DateTime" />
        </InsertParameters>
    </asp:SqlDataSource>
    <asp:SqlDataSource ID="categoriesDataSource" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionString %>"
        SelectCommand="SELECT [CategoryID], [Name] FROM [Categories] ORDER BY [Name]">
    </asp:SqlDataSource>
    <asp:SqlDataSource ID="maxPictureIDDataSource" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionString %>"
        SelectCommand="SELECT MAX (PictureID)&#13;&#10;FROM Pictures&#13;&#10;WHERE UserId = @UserId">
        <SelectParameters>
            <asp:ControlParameter ControlID="UserIdValue" Name="UserId" PropertyName="Text" />
        </SelectParameters>
    </asp:SqlDataSource>
    <br />
    <asp:DetailsView ID="dvPictureInsert" runat="server" AutoGenerateRows="False" DataKeyNames="PictureID"
        DataSourceID="pictureDataSource" DefaultMode="Insert" Height="50px" Width="125px">
        <Fields>
            <asp:BoundField DataField="PictureID" HeaderText="PictureID" InsertVisible="False"
                ReadOnly="True" SortExpression="PictureID" />
            <asp:TemplateField HeaderText="CategoryID" SortExpression="CategoryID">
                <EditItemTemplate>
                    <asp:TextBox ID="TextBox1" runat="server" Text='<%# Bind("CategoryID") %>'></asp:TextBox>
                </EditItemTemplate>
                <InsertItemTemplate>
                    <asp:DropDownList ID="DropDownList1" runat="server" DataSourceID="categoriesDataSource"
                        DataTextField="Name" DataValueField="CategoryID" SelectedValue='<%# Bind("CategoryID") %>' AppendDataBoundItems="True">
                        <asp:ListItem>--None--</asp:ListItem>
                    </asp:DropDownList>
                </InsertItemTemplate>
                <ItemTemplate>
                    <asp:Label ID="Label1" runat="server" Text='<%# Bind("CategoryID") %>'></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Title" SortExpression="Title">
                <EditItemTemplate>
                    <asp:TextBox ID="TextBox2" runat="server" Text='<%# Bind("Title") %>'></asp:TextBox>
                </EditItemTemplate>
                <InsertItemTemplate>
                    <asp:TextBox ID="TextBox1" runat="server" Text='<%# Bind("Title") %>'></asp:TextBox><br />
                    <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="TextBox1"
                        Display="Dynamic" ErrorMessage="RequiredFieldValidator" ValidationGroup="PictureAdd"></asp:RequiredFieldValidator>
                </InsertItemTemplate>
                <ItemTemplate>
                    <asp:Label ID="Label2" runat="server" Text='<%# Bind("Title") %>'></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Description" SortExpression="Description">
                <EditItemTemplate>
                    <asp:TextBox ID="TextBox3" runat="server" Text='<%# Bind("Description") %>'></asp:TextBox>
                </EditItemTemplate>
                <InsertItemTemplate>
                    <asp:TextBox ID="TextBox2" runat="server" Text='<%# Bind("Description") %>'></asp:TextBox><br />
                    <asp:RequiredFieldValidator ID="RequiredFieldValidator2" runat="server" ControlToValidate="TextBox2"
                        Display="Dynamic" ErrorMessage="RequiredFieldValidator" ValidationGroup="PictureAdd"></asp:RequiredFieldValidator>
                </InsertItemTemplate>
                <ItemTemplate>
                    <asp:Label ID="Label3" runat="server" Text='<%# Bind("Description") %>'></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:CommandField ShowInsertButton="True" />
        </Fields>
    </asp:DetailsView>
    <br />
    <br />
</asp:Content>