C# Bitmap rotate with transparent image

I tried two png images as one on another. Those are rotating through a code as follows.

private Bitmap RotateImage(Bitmap bmp, float angle)
        {
            Bitmap rotatedImage = new Bitmap(bmp.Width, bmp.Height);
            rotatedImage.SetResolution(bmp.HorizontalResolution, bmp.VerticalResolution);

            using (Graphics g = Graphics.FromImage(rotatedImage))
            {
                g.TranslateTransform(bmp.Width / 2, bmp.Height / 2);
                g.RotateTransform(angle);
                g.TranslateTransform(-bmp.Width / 2, -bmp.Height / 2);
                g.DrawImage(bmp, new Point(0, 0));            }

            return rotatedImage;
        }

But, even select the background of the picture as transparent, shows the background color of the form as the background color of the image of the front image. I tried to add a code to paint control as

e.Graphics.DrawImage(bmp, 50, 50, 100, 100);

But when the function returns the new bitmap. It's not working. How do I do this? Current Result.

Here's the complete code:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.IO;
using System.Linq;
using System.Reflection;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Testing
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        Bitmap bmp;
        Bitmap bmp1;
        Bitmap bmp2;
        public float angle = 10;
        public float rangle = 0;
        PaintEventArgs e;

        private void btn_Rotate_Click(object sender, EventArgs e)
        {
            timer1.Start();
        }

        private Bitmap RotateImage(Bitmap bmp, float angle)
        {
            Bitmap rotatedImage = new Bitmap(bmp.Width, bmp.Height);
            rotatedImage.SetResolution(bmp.HorizontalResolution, bmp.VerticalResolution);

            using (Graphics g = Graphics.FromImage(rotatedImage))
            {
                // Set the rotation point to the center in the matrix
                g.TranslateTransform(bmp.Width / 2, bmp.Height / 2);
                // Rotate
                g.RotateTransform(angle);
                // Restore rotation point in the matrix
                g.TranslateTransform(-bmp.Width / 2, -bmp.Height / 2);
                // Draw the image on the bitmap
                g.DrawImage(bmp, new Point(0, 0));

                bmp.MakeTransparent();
            }

            return rotatedImage;
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            // C:\Users\samsng\Desktop\left_right.png
            try
            {
                bmp = (Bitmap)Bitmap.FromFile(Path.Combine(Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location), "Pics/123.png"));
                bmp1 = (Bitmap)Bitmap.FromFile(Path.Combine(Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location), "Pics/3.png"));
                bmp2 = (Bitmap)Bitmap.FromFile(Path.Combine(Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location), "Pics/4.png"));

                pictureBox2.SizeMode = PictureBoxSizeMode.StretchImage;
                pictureBox2.Image = bmp;
                pictureBox1.SizeMode = PictureBoxSizeMode.StretchImage;
                pictureBox1.Image = bmp1;                
                pictureBox3.SizeMode = PictureBoxSizeMode.StretchImage;
                pictureBox3.Image = bmp2;
            }
            catch (FileNotFoundException)
            {
                MessageBox.Show("File Not Found Exception");
            }
            lblNum.Visible = false;
        }
        int time = 01;
        private void timer1_Tick(object sender, EventArgs e)
        {
            int i = 1;
            if (bmp != null)
            {
                pictureBox1.Image = RotateImage(bmp1, angle);
                pictureBox3.Image = RotateImage(bmp2, rangle);
                pictureBox2.Image = RotateImage(bmp, angle);
                angle += 10;
                rangle -= 10;
            }
            i++;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            lblNum.Visible = true;
            timer2.Start();
        }

        private void timer2_Tick(object sender, EventArgs e)
        {
            if (time < 10)
            {
                lblNum.Text = "0" + time.ToString();
            }
            else
            {
                lblNum.Text = time.ToString();
            }
            time++;
            if (time == 99)
            {
                time = 01;
            }
        }

        private void pictureBox3_Paint(object sender, PaintEventArgs e)
        {
            //MessageBox.Show(e.ToString());
        }

        private void pictureBox1_Paint(object sender, PaintEventArgs e)
        {
            //e.Graphics.DrawImage(bmp1, 50, 50, 100, 100);
        }
    }
}