"The remote server returned an error (502) Bad Gateway" while downloading image file from url

I am trying to download image from website, but it gives me error i.e. (The remote server returned an error: (502) Bad Gateway). I search a lot on internet but didn't get exact solution. This code is working on some website but on few website it is showing above error. Please check my below code...

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    Dim filename As String = Application.StartupPath
    If Not filename.EndsWith("\") Then filename &= "\"
    txtLocalFile.Text = "D:\a.jpg" 'filename & " "
    txtRemoteFile.Text = "http://ift.tt/1ErQ2Js"
End Sub


Private Sub btnDownload_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnDownload.Click
    Application.DoEvents()
    Try
        Dim web_client As WebClient = New WebClient

        Dim wp = New WebProxy("proxy ip", proxy_port)
        wp.Credentials = New NetworkCredential("uname", "password", "domainname")
        web_client.Proxy = wp
        web_client.DownloadFile(txtRemoteFile.Text, txtLocalFile.Text)

        MessageBox.Show("Done")
    Catch ex As Exception

    End Try

    End Sub

Can you please tell me which part of code is missing or i have to change..