Thread , semaphore and outofmemory exception in vb.net

Module Module1

Dim TotalThread As Integer = 3
Dim SemaphoreCount As Integer = 2
Dim Threads As Thread() = New Thread(TotalThread - 1) {}
Dim semaph As New Semaphore(SemaphoreCount, SemaphoreCount)

Private baseUrl As String = "http://www.domain.com"
Private m_SavePath As String = "c:\test\"

Public Structure ParameterList
    Public ImageUrl As String
    Public FileName As String
    Public objSemaphore As Semaphore
End Structure

<MTAThread> _
Sub Main()
   checkImage() 
End Sub

Private Function checkImage() As Boolean

    Dim _param As New ParameterList

    Dim dbConn As New SqlClient.SqlConnection("Data Source=localhost\sqlexpress;Initial Catalog=db;Persist Security Info=True;User ID=username;Password=password")
    dbConn.Open()
    Dim cmd As New SqlCommand
    cmd.Connection = dbConn
    cmd.CommandType = CommandType.Text
    cmd.CommandText = "SELECT  LinkImg.codice , LinkImg.foto , LinkImg.Zoom FROM LinkImg WHERE codice NOT IN (select codice from _PRODOTTI_IMMAGINI)"

    Dim dtr As SqlDataReader

    Try
        dtr = cmd.ExecuteReader(CommandBehavior.CloseConnection)

        If dtr.HasRows Then
            While dtr.Read

                Dim t1 As New Thread(New ParameterizedThreadStart(AddressOf DownloadRemoteImageFile))
                Dim t2 As New Thread(New ParameterizedThreadStart(AddressOf DownloadRemoteImageFile))

                _param.ImageUrl = dtr("zoom")
                _param.FileName = "img_zoom_" & dtr("codice").ToString.Trim & ".jpg"
                _param.objSemaphore = semaph

                t1.IsBackground = True
                t1.Start(_param)

                _param.ImageUrl = dtr("foto")
                _param.FileName = "img_Small_" & dtr("codice").ToString.Trim & ".jpg"
                _param.objSemaphore = semaph

                t2.IsBackground = True
                t2.Start(_param)

                t1 = Nothing
                t2 = Nothing

            End While
        End If

    Catch ex As Exception
        Throw ex
    Finally
        dbConn.Close()
    End Try

    Return True
End Function

Public Sub DownloadRemoteImageFile(ByVal param As Object)

    Dim l_SemaPhore As Semaphore = DirectCast(param.objSemaphore, Semaphore)

    If l_SemaPhore.WaitOne(100, False) Then
        Try
            Dim imageUrl As String = baseUrl & param.ImageUrl

            Dim imageBytes As Byte()
            Dim imageRequest As HttpWebRequest = DirectCast(WebRequest.Create(imageUrl), HttpWebRequest)
            Dim imageResponse As HttpWebResponse = TryCast(imageRequest.GetResponse(), HttpWebResponse)

            If (imageResponse.StatusCode = HttpStatusCode.OK OrElse imageResponse.StatusCode = HttpStatusCode.Moved OrElse imageResponse.StatusCode = HttpStatusCode.Redirect) AndAlso imageResponse.ContentType.StartsWith("image", StringComparison.OrdinalIgnoreCase) Then
                Dim responseStream As Stream = imageResponse.GetResponseStream()

                Using br As New BinaryReader(responseStream)
                    imageBytes = br.ReadBytes(500000)
                    br.Close()
                End Using

                responseStream.Close()
                imageResponse.Close()

                Dim fs As FileStream
                Dim bw As BinaryWriter

                Try
                    fs = New FileStream(m_SavePath & param.FileName.Replace("/", "-"), FileMode.Create)
                    bw = New BinaryWriter(fs)

                    bw.Write(imageBytes)
                Catch ex As Exception

                Finally
                    If fs IsNot Nothing Then fs.Close()
                    If bw IsNot Nothing Then bw.Close()
                End Try

                Thread.Sleep(200)

            End If

        Finally
            l_SemaPhore.Release()
        End Try
    End If
End Sub
End Module