Prova così

codice:
<%@ Page Language="VB" %>
<%@ Import Namespace="System.IO" %>
<%@ Import Namespace="System.Drawing" %>
<%@ Import Namespace="System.Drawing.Imaging" %>
<%@ Import Namespace="System.Drawing.Drawing2D" %>
<%
Dim prova
prova="milano"
%>
<script runat="server">

Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs)

'Inizializzo lo stream in memoria
Dim MemStream As New MemoryStream()

'Creo una nuova bitmap
Dim bitmap As New System.Drawing.Bitmap(468, 60, PixelFormat.Format32bppArgb)
Dim graphic As Graphics = Graphics.FromImage(bitmap)

Dim penna As New Pen(Color.FromArgb(255, 255, 255), 10)
Dim pennello As SolidBrush = New SolidBrush(Color.FromArgb(206, 0, 6))

'Pulisco l'immagine e aggiungo l'antialias (anche per il testo)
graphic.Clear(Color.FromArgb(20, 100, 0, 255))
graphic.SmoothingMode = SmoothingMode.AntiAlias
graphic.TextRenderingHint = System.Drawing.Text.TextRenderingHint.AntiAlias

'Disegno l'immagine
'graphic.DrawLine(penna, 7, 80, 110, 80)
graphic.DrawString("Sicilia 4 notti da € 230", New Font("Tahoma", 24, FontStyle.Bold), pennello, New PointF(2, 3))
graphic.DrawString("prova", New Font("verdana", 8, FontStyle.Bold), pennello, New PointF(2, 45))

Response.ContentType = "image/png"

'Salvo nello stream l'immagine e poi la invio in output
bitmap.Save(MemStream, ImageFormat.Png)
bitmap.Save(Server.MapPath("immagine.png"),ImageFormat.Png)
MemStream.WriteTo(Response.OutputStream)

bitmap.Dispose()

End Sub

</script>