Web Forms :: Make "UserId" Equal To The "logged-in User" In Vb Code Behind

I need to insert a declaration in the codebehind function below that will make the variable "UserId" equal to the "logged-in user". In what way do I need to change the code below (elsewhere in the project, I successfully make reference to basically the same code and make the UserId equal to logged-in user by using this handler expression inside an image control: ImageUrl ='<%# Eval("UserId","Image.aspx?UserId={0}")%>'...But in the case below, the function is not fired by this expression, but rather by CustomValidator).

Sub ValidateImage(ByVal source As Object, ByVal args As System.Web.UI.WebControls.ServerValidateEventArgs)
If Request.QueryString("UserId") IsNot Nothing Then
' Get the connection string from Web.config.
Using conn As New SqlConnection(ConfigurationManager.ConnectionStrings("MyDatabaseConnectionString").ToString())
' Create a command object.
Dim cmd As New SqlCommand()
' Assign the connection to the command.
cmd.Connection = conn
' Appendy the parameter to the SqlCommand and set value.
cmd.Parameters.Add("@UserId", SqlDbType.UniqueIdentifier).Value = Guid.Parse(Request.QueryString("UserId"))
' Set the command text
' SQL statement or the name of the stored procedure.
cmd.CommandText = "SELECT Avatar FROM aspnet_Membership WHERE UserId = @UserId AND Avatar IS NOT NULL"
' Set the command type
' CommandType.Text for ordinary SQL statements;
' CommandType.StoredProcedure for stored procedures.
cmd.CommandType = CommandType.Text
' Open the connection.
conn.Open()
' Convert the returned result to a bytes array.
Dim bytes As Byte() = DirectCast(cmd.ExecuteScalar(), Byte())
If bytes IsNot Nothing Then
' Set the HTTP MIME type of the output stream.
Response.ContentType = "image/jpeg"
' Write a string of Binary characters to the HTTP
' output stream.
Response.BinaryWrite(bytes)
' Send all currently buffered output to the client.
Response.End()
Else
args.IsValid = False
End If
End Using
End If
End Sub