Почему бы не сделать предположим так:
- На странице у тя имеется картика 
<img src="image.aspx?<параметры твоей карты>" />
- А в image.aspx формируешь и выводишь изображение на событии Page_Load.

// Вот как можно вывести jpeg
Respone.Clear();
Response.ContentType = "image/jpeg";
Response.BinaryWrite(imageData);
// в imageData твоя итоговая картинка
Response.End();

 bmp.Save(Response.OutputStream, ImageFormat.Jpeg)

Response.BinaryWrite(CType(DR( 0 ), Byte()))

Dim bmp As Bitmap = NewImageBlaBla()
Response.ContentType = "image/jpeg"
bmp.Save(Response.OutputStream, ImageFormat.Jpeg)

Partial Class SQL_Image
    Inherits System.Web.UI.UserControl

    Private _ConnectionStringsName As String = ""
    Private _SelectStatement As String = ""

    Public Property ConnectionStringsName() As String
        Get
            ConnectionStringsName = _ConnectionStringsName
        End Get
        Set(ByVal value As String)
            _ConnectionStringsName = value
        End Set
    End Property

    Public Property SelectStatement() As String
        Get
            SelectStatement = _SelectStatement
        End Get
        Set(ByVal value As String)
            _SelectStatement = value
        End Set
    End Property

    Public Sub Show(ByVal ConnectionStringsName As String, ByVal SelectStatement As String)
        Image1.ImageUrl = "SQLImage.aspx?ConnectionStringsName=" & ConnectionStringsName & "&SelectStatement=" & SelectStatement
    End Sub

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        If _ConnectionStringsName <> "" And _SelectStatement <> "" Then Image1.ImageUrl = "SQLImage.aspx?ConnectionStringsName=" & _ConnectionStringsName & "&SelectStatement=" & _SelectStatement
    End Sub
End Class

Imports System.Data.SqlClient
Partial Class SQLImage
    Inherits System.Web.UI.Page
    Dim CN As SqlConnection, CMD As SqlCommand, DR As SqlDataReader
    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        CN = New SqlConnection(ConfigurationManager.ConnectionStrings(Request.QueryString("ConnectionStringsName")).ConnectionString)
        CN.Open()
        CMD = New SqlCommand(Request.QueryString("SelectStatement"), CN)
        DR = CMD.ExecuteReader
        If DR.Read Then
            If DR( 0 ).Length >  0  Then   'в SQL-поле NULL
                Response.ContentType = "image/bmp"
                Response.BinaryWrite(CType(DR( 0 ), Byte()))
            End If
        End If
        DR.Close()
        CN.Close()
    End Sub
End Class

Я ,признаться, долго долбался сам с этим