Примерно так:
var filePath = @"D:\Pictures\Backgrounds\abc.jpg";
Bitmap bitmap = null;
// Create from a stream so we don't keep a lock on the file.
using (var stream = File.OpenRead(filePath))
{
bitmap = (Bitmap)Bitmap.FromStream(stream);
}
using (bitmap)
using (var graphics = Graphics.FromImage(bitmap))
using (var font = new Font("Arial", 20, FontStyle.Regular))
{
// Do what you want using the Graphics object here.
graphics.DrawString("Hello World!", font, Brushes.Red, 0, 0);
// Important part!
bitmap.Save(filePath);
}