public static Bitmap ImageRotate(Bitmap bmp, float angle, Color bkColor)
        {
            int w = bmp.Width + 2;
            int h = bmp.Height + 2;

            PixelFormat pf;

            if (bkColor == Color.Transparent)
            {
                pf = PixelFormat.Format32bppArgb;
            }
            else
            {
                pf = bmp.PixelFormat;
            }

            Bitmap tmp = new Bitmap(w, h, pf);
            Graphics g = Graphics.FromImage(tmp);
            g.Clear(bkColor);
            g.DrawImageUnscaled(bmp, 1, 1);
            g.Dispose();

            GraphicsPath path = new GraphicsPath();
            path.AddRectangle(new RectangleF(0f, 0f, w, h));
            Matrix mtrx = new Matrix();
            mtrx.Rotate(angle);
            RectangleF rct = path.GetBounds(mtrx);

            Bitmap dst = new Bitmap((int)rct.Width, (int)rct.Height, pf);
            g = Graphics.FromImage(dst);
            g.Clear(bkColor);
            g.TranslateTransform(-rct.X, -rct.Y);
            g.RotateTransform(angle);
            g.SmoothingMode = SmoothingMode.HighQuality;
            g.CompositingQuality = CompositingQuality.HighQuality;
            g.TextRenderingHint = System.Drawing.Text.TextRenderingHint.AntiAlias;

            g.InterpolationMode = InterpolationMode.HighQualityBicubic;
            g.PixelOffsetMode = PixelOffsetMode.HighQuality;
            g.DrawImageUnscaled(tmp, 0, 0);
            g.Dispose();
            tmp.Dispose();

            return dst;
        }

            var bmp = new Bitmap(@"C:\Users\admin\Desktop\1.png");
            var newBmp = ImageRotate(bmp, -15, Color.Transparent);
            newBmp.Save(@"C:\Users\admin\Desktop\new1.png");