How to change Image Thumbnail

string strPath="c:\\bb.jpg",strMainPicPath="c:\\aa.jpg" ;
Image imgBase;
imgBase = Image.FromFile(strMainPicPath);

SolidBrush brush = new SolidBrush(cWhite);
Font fntB = new Font("tahoma", 8, FontStyle.Bold);
System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(imgBase);
g.DrawString("Test String", fntB, brush, 100, 100);
g.Dispose();

imgBase.Save(strPath, System.Drawing.Imaging.ImageFormat.Jpeg);
imgBase.Dispose();