just to add to the answers - if this refers to a file that physically exists on disk, then why not let fileinfo do the work for you?
var path = @"C:\Users\Esat\Desktop\BilimResimler\1620855_759701257391419_1132489417_n.jpg";
System.IO.FileInfo myImageFile = new System.IO.FileInfo(path);
Console.WriteLine(myImageFile.Name); // gives 1620855_759701257391419_1132489417_n.jpg