Compiler Error Message CS0120 - Object Reference Is Required For Nonstatic Field

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/TheDogCropped.jpg";
protected void Page_Load(object sender, System.EventArgs e)
{
if (!Page.IsPostBack)
{
loadPhoto(ORIG_SAMPLE_PHOTO_URL);
}
else {
loadPhoto(CROPPED_SAMPLE_PHOTO_URL);
btnCrop.Visible = !btnCrop.Visible;
}
}