How to 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). Thanks very much.
 
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. ...