I'm sure you've solved it by now.  It is an ASP.Net public property, see this link:  http://msdn.microsoft.com/en-us/library/system.web.ui.page.maintainscrollpositiononpostback%28v=vs.110%29.aspx?cs-save-lang=1&cs-lang=vb#code-snippet-1

 Select C# if that's your dev language, in this case I needed to find it for VB.

In the Page Load event in question, just start typing "maintain " and it should show up in IntelleSense as you'll see in this snippet:

                _Thumb.Save(Response.OutputStream, codecEncoder, encodeParams)
                'Else
                '_Thumb.Save(Response.OutputStream, imgFormat)
                'End If
            End If

            _OrginalImg.Dispose()
            _Thumb.Dispose()
            MaintainScrollPositionOnPostBack = True

        End Sub

        Private Function MakeThumbnail(ByVal OrginalImg As System.Drawing.Image, ByVal width As Integer, ByVal height As Integer, ByRef scaleFactor As Single) As System.Drawing.Image
            If width > 0 OrElse height > 0 Then
                Dim imgHeight, imgWidth As Integer

                imgHeight = OrginalImg.Height
                imgWidth = OrginalImg.Width