How to change image Name during saving image using C# in ASP.Net

html code

<div class="Row100">
    <div class="txtleft">Student Photo :</div>
    <div class="txtmid1">
        <div class="up_img1">
            <asp:Image ID="stuphoto" runat="server" />
        </div>
    </div>
    <div class="txtmid2">Student Signature :</div>
    <div class="txtright">
        <div class="up_sign1">
            <asp:Image ID="stusign" runat="server" />
        </div>
    </div>
</div>

<div class="Row100">
    <div class="leftcol550">
        <asp:FileUpload ID="stuupphoto" runat="server" CssClass="Button" />
    </div>
    <div class="rightcol">
        <asp:FileUpload ID="stuupsign" runat="server" CssClass="Button" />
    </div>
</div>

<div class="Row">
    <div class="txtmid1">
         <asp:Button ID="btnuploadimg" runat="server" CssClass="Button" Text="Upload & Next" OnClick="btnuploadimg_Click" />
    </div>
</div>

page.aspx.cs

////saving file in the physical folder;

string virtualFolder = "~/Stu_KYC/Photos/";
string virtualFolder1 = "~/Stu_KYC/Sign/";

FileUpload FileUpload1 = stuupphoto;
FileUpload FileUpload2 = stuupsign;

string file_ext = Path.GetExtension(stuupphoto.PostedFile.FileName);
string file_ext1 = Path.GetExtension(stuupsign.PostedFile.FileName);

stuupphoto.SaveAs(Server.MapPath(virtualFolder) + "\\" + lblregdno.Text + "" + file_ext);
stuupsign.SaveAs(Server.MapPath(virtualFolder1) + "\\" + lblregdno.Text + "S" + file_ext1);

string photo = lblregdno.Text + "" + file_ext;
string sign = lblregdno.Text + "S" + file_ext1;

string physicalFolder = Server.MapPath(virtualFolder);
string physicalFolder1 = Server.MapPath(virtualFolder1);

FileUpload1.SaveAs(physicalFolder + FileUpload1.FileName);
FileUpload2.SaveAs(physicalFolder1 + FileUpload2.FileName);

stuphoto.Visible = true;
stusign.Visible = true;
stuphoto.ImageUrl = Server.MapPath(virtualFolder) + @"\" + lblregdno.Text + "" + file_ext;
stusign.ImageUrl = Server.MapPath(virtualFolder1) + @"\" + lblregdno.Text + "" + file_ext1;

SqlConnection cnn = new SqlConnection();
cnn.ConnectionString = System.Configuration.ConfigurationManager.ConnectionStrings["care_cnn"].ConnectionString;
cnn.Open();

SqlCommand cmd = new SqlCommand("insert into eccadmin_kycimages values('" + lbl_email.Text + "','" + lblregdno.Text + "','" + photo + "','" + sign + "','" + DateTime.Now + "')", cnn);
cmd.ExecuteNonQuery();
cnn.Close();

Please help me out.