You can set the background image in textbox and write the below code in textbox text change event:-

private void txtSearch_TextChanged(object sender, TextChangedEventArgs e)
        {
            if (txtSearch.Text != "")
            {
                this.txtSearch.Background = null;
            }
            else
            {
                ImageBrush mybrush = new ImageBrush();
                mybrush.ImageSource = new BitmapImage(new Uri("Image Path"));
                this.txtSearch.Background = mybrush;
            }
        }

thanks