The code demonstrated the following:

1. Limited the selection of JPG and PNG files only
2. Only 1 file can be selected
3. Upload the file with the corresponding file name
4. How to get the file data inside PHP

OpenFileDialog dialog = new OpenFileDialog();
dialog.Filter = "Image Files (.jpg,.png)|*.JPG;*.jpg;*.PNG;*.png";
dialog.FilterIndex = 1;
bool? userClickedOK = _imageDialog.ShowDialog();

// Process input if the user clicked OK.
if (userClickedOK == true)
{
    UploadFile(dialog.File.Name, dialog.File.OpenRead());
}

private void UploadFile(string filename, Stream data)
{
    UriBuilder ub = new UriBuilder(UPLOAD_PATH);
    ub.Query = string.Format("filename={0}", filename);

    WebClient c = new WebClient();
    c.OpenWriteCompleted += (sender, e) =>
    {
        PushData(data, e.Result);
        e.Result.Close();
        data.Close();
    };
    c.OpenWriteAsync(ub.Uri, "Post");
}

private void PushData(Stream input, Stream output)
{
    byte[] buffer = new byte[4096];
    int bytesRead;

    while ((bytesRead = input.Read(buffer, 0, buffer.Length)) != 0)
    {
        output.Write(buffer, 0, bytesRead);
    }
}

$filepath = $_REQUEST['filename'];
$handle = fopen("php://input", "r");
$file = fopen($filepath,"w");

if($file != null && $handle != null)
{
        while ($data = fread($handle, 8192)) fwrite($file, $data);
        fclose($handle);
        fclose($file);
}