Hi  KatieNgoc,

Refer below link for showing string to image and also refer below sample for drawing image on textbox.

ASP.Net: Convert Text String to Image using C# and VB.Net

HTML

Name :
<asp:TextBox runat="server" ID="txtName" />
<br />
<asp:Button Text="Print" runat="server" OnClick="Signature" />

Namespaces

C#

using System.Drawing;
using System.Drawing.Imaging;

VB.Net

Imports System.Drawing
Imports System.Drawing.Imaging

Code

C#

protected void Signature(object sender, EventArgs e)
{
    Bitmap bitmap = new Bitmap(250, 100, System.Drawing.Imaging.PixelFormat.Format64bppArgb);
    Graphics graphics = Graphics.FromImage(bitmap);
    graphics.Clear(Color.White);
    graphics.DrawString(txtName.Text, new Font("Informal Roman", 25, FontStyle.Regular), new SolidBrush(Color.FromArgb(255, 0, 0)), new PointF(0.4F, 2.4F));
    bitmap.Save(Response.OutputStream, ImageFormat.Jpeg);
    bitmap.Dispose();
}

VB.Net

Protected Sub Signature(ByVal sender As Object, ByVal e As EventArgs)
    Dim bitmap As Bitmap = New Bitmap(250, 100, System.Drawing.Imaging.PixelFormat.Format64bppArgb)
    Dim graphics As Graphics = Graphics.FromImage(bitmap)
    graphics.Clear(Color.White)
    graphics.DrawString(txtName.Text, New Font("Informal Roman", 25, FontStyle.Regular), New SolidBrush(Color.FromArgb(255, 0, 0)), New PointF(0.4F, 2.4F))
    bitmap.Save(Response.OutputStream, ImageFormat.Jpeg)
    bitmap.Dispose()
End Sub

Screenshot