thanks this code worked for me !
string path = GetImgURL(Picture_id);
System.Drawing.Image _sourceImage = System.Drawing.Image.FromFile(Server.MapPath(path));
System.Drawing.Image _thumbImage = this.CreateThumbnail(_sourceImage, 40, 40);
_thumbImage.Save( Server.MapPath("~") +"\\Uploads/Day.gif", ImageFormat.Jpeg)