copying image files from one folder to another

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ASP.NET

hi,
wonder if someone can please guide me on this.
i m trying to copy image files from one location to another.
the path of the original file is stored as a value in a cell in a gridview.

so far the code i have tried is like so:

for (i = 0; i <= GridView1.Rows.Count-1 ; i++)
           {

               string filepath = GridView1.Rows[i].Cells[2].Text;
               string filename = GridView1.Rows[i].Cells[3].Text; 
           System.IO.File.Copy(filepath, "~/" + foldername + "/" + filename);
          }

on debugging the value in filepath is "~/carpics/back.bmp" and The value in filename is "back.bmp"
At the file.copy it gives me an error "Could not find a part of the path '~/carpics/back.bmp'."

Please help me fix this or if there is another way to copy image files.
please guide.
thanks.