try by saving new file and deleting the old file..
if (!File.Exists(Server.MapPath("~/ItemImage/" + Convert.ToString(Request.QueryString["compid"]) + "/" + Convert.ToString(Request.QueryString["offID"]) + "/" + itemImgName)))
{
string newImgName = Convert.ToString(itemID);
FileUpload file = new FileUpload();
file.SaveAs(Server.MapPath("~/ItemImage/" + Convert.ToString(Request.QueryString["compid"]) + "/" + Convert.ToString(Request.QueryString["offID"]) + "/" + newImgName + "." + fileExt)));
FileInfo MyFile = new FileInfo(Server.MapPath("~/ItemImage/" + Convert.ToString(Request.QueryString["compid"]) + "/" + Convert.ToString(Request.QueryString["offID"]) + "/" + itemImgName + "." + fileExt));
MyFile.Delete();
}