Compiler Error Message: CS0120: An object reference is required for the nonstatic field, method, or property 'System.Web.UI.Page.Request.get'

I usually only write in ASP, so I am not very familiar with this script. Can anyone tell me how to how to request a querystring so I can use it to pull the image location. Like this.

PhotoCropper.aspx?userid=1090&picid=47

private const string ORIG_SAMPLE_PHOTO_URL = @"/PhotoCropperC/photos/1090/47.jpg";
 
I have tried just using Request.QueryString["userid"];, but I get the compiler error.
Here is my code:
public partial class PhotoCropper : System.Web.UI.Page
{

    //Make sure you give the IUSR_MachineName permissions to the directory below
    private const string ORIG_SAMPLE_PHOTO_URL =  @"/PhotoCropperC/photos/TheDog.jpg";
   //private const string ORIG_SAMPLE_PHOTO_URL = "~/photos/TheDog.jpg";
    private const string CROPPED_SAMPLE_PHOTO_URL = @"d:/inetpub/sites/dc/cropper/TheD ...