Me respondo a mi mismo, aki está la solución
Código:
<script language="VB" runat="server" Debug="true">
Sub Page_Load(Sender As Object, E As EventArgs)
Dim originalimg, thumb As System.Drawing.Image
Dim FileName As String
Dim inp As New IntPtr()
Dim width, height As Integer
Dim rootpath As String
rootpath = Server.MapPath("/") ' Get Root Application Folder
FileName = Server.MapPath(Request.QueryString("FileName")) ' Root Folder + FileName
Try
originalimg = originalimg.FromFile(FileName) ' Fetch User Filename
Catch
originalimg = originalimg.FromFile(rootpath & "error.gif") ' Fetch error.gif
End Try
' Get width using QueryString.
If Request.QueryString("width") = Nothing Then
width = originalimg.Width ' Use original Width.
ElseIf Request.QueryString("width") = 0 Then ' Assign default width of 100.
width = 100
Else
width = Request.QueryString("width") ' Use User Specified width.
End If
' Get height using QueryString.
If Request.QueryString("height") = Nothing Then
height = originalimg.Height ' Use original Height.
ElseIf Request.QueryString("height") = 0 Then ' Assign default height of 100.
height = 100
Else
height = Request.QueryString("height") ' Use User Specified height.
End If
thumb = originalimg.GetThumbnailImage(width, height, Nothing, inp)
' Sending Response JPEG type to the browser.
Response.ContentType = "image/jpeg"
Dim NombreArchivo as String
NombreArchivo = "thumb_" & right(FileName,len(FileName) - instrrev(FileName,"\"))
Dim imagenpequena as String
imagenpequena = Server.MapPath("/") & "..\imagenes\pruebas\" & NombreArchivo
thumb.Save(imagenpequena, System.Drawing.Imaging.ImageFormat.Jpeg) 'Change jpeg word
' Disposing the objects.
originalimg.Dispose()
thumb.Dispose()
End Sub
</script>
Espero que os sirva, por cierto para que no os de error el usuario asp.net tiene que tener permisos de escritura.
Saludos