MessageBox is the subroutine that is called if the select case fails and
passes string back to it and display a client side alertbox.
Here's the full code:

Private Sub MessageBox(ByVal strMsg As String)
        ' define a javascript alertbox containing
        ' the string passed in as argument
        ' create a new label

        Dim lbl As New Label()
        ' add the javascript to fire an alertbox to the label and
        ' add the string argument passed to the subroutine as the
        ' message payload for the alertbox
        lbl.Text = "<script type=""text/javascript"">" &
Environment.NewLine & "window.alert('" + strMsg + "')</script>"
        ' add the label to the page to display the alertbox
        Page.Controls.Add(lbl)

    End Sub

Private Sub FormView1_ItemInserting(ByVal sender As Object, ByVal e As
System.Web.UI.WebControls.FormViewInsertEventArgs) Handles
FormView1.ItemInserting
        'Reference the FileUpload control
        Dim UploadedFilePic As FileUpload =
CType(FormView1.FindControl("fileUploadProdPict"), FileUpload)

        'Make sure we are dealing with a JPG,  GIF or PNG file
        Dim extensionPic As String =
Path.GetExtension(UploadedFilePic.PostedFile.FileName).ToLower()
        Dim filenamePic As String =
Path.GetFileName(UploadedFilePic.PostedFile.FileName)
        Dim MIMETypePic As String = Nothing

        Select Case extensionPic
            Case ".gif"
                MIMETypePic = "image/gif"
            Case ".jpg", ".jpeg", ".jpe"
                MIMETypePic = "image/jpeg"
            Case ".png"
                MIMETypePic = "image/png"

            Case Else
                MessageBox("Product picture:\r\nOnly .gif, .jpg and
.png file types\r\ncan be selected for upload to database.")
                UploadedFilePic.Focus()
                e.Cancel = True
                Exit Sub
        End Select

        'Specify the values for the MIMEType and ImageData parameters
        e.Values("productPicMimeType") = MIMETypePic
        'Load FileUpload's InputStream into Byte array
        Dim
imageBytesPic(UploadedFilePic.PostedFile.InputStream.Length) As Byte
        UploadedFilePic.PostedFile.InputStream.Read(imageBytesPic, 0,
imageBytesPic.Length)
        e.Values("productPic") = imageBytesPic
        e.Values("productPicFileName") = filenamePic
    End Sub

Therefore if the item Inserting event fails, at the select case stage,
the user is prompted to try again. I know its not the technically
correct solution but it does work. Ideally i'd like full client side
validation before the item inserting event is fired but javascript is
not my strong suit!