Downgrading image causes the image rotation

I am uploading images to my AWS S3 storage using asp.net website. 
I have a process before uploading the image. I downgrade the image quality before uploading it to the cloud. 
When I give it an image that its width is bigger than its height, the process works without any problem, but when I try it on an image that its height is bigger than its width, it rotates the image -90 degrees.

public void image_Upload(){
 Image fixed_image = ResizeImage(Path.GetFileName(FileUpload1.FileName), myimg, new Size(800, 500));
}
public Image ResizeImage(string fileName, Image imgToResize, Size size)
        {
            int sourceWidth = imgToResize.Width;
            int sourceHeight = imgToResize.Height;

            float nPercentW = (size.Width / (float)sourceWidth);
            float nPercentH = (size.Height / (float)sourceHeight);

            float nPercent = nPercentH < nPercentW ? nPercentH : nPercentW;
           
          
            var destWidth = (int)(sourceWidth * nPercent);
            var destHeight = (int)(sourceHeight * nPercent);

            var src = imgToResize;

            using (var dst = new Bitmap(destWidth, destHeight, imgToResize.PixelFormat))
            {
               
                    dst.SetResolution(imgToResize.HorizontalResolution, imgToResize.VerticalResolution);
                 using (var g = Graphics.FromImage(dst))
                {
                    var mime = Path.GetExtension(fileName).ToLower();
                    ImageFormat format;
                    if (mime == ".bmp" || mime == ".png")
                    {
                        //convert all gif to png, better resize quality
                        g.SmoothingMode = SmoothingMode.AntiAlias;
                        g.InterpolationMode = InterpolationMode.HighQualityBicubic;
                        g.DrawImage(src, 0, 0, dst.Width, dst.Height);
                        format = ImageFormat.Png;
                    }
                    else
                    {
                        //jpeg
                        g.CompositingQuality = CompositingQuality.HighQuality;
                        g.InterpolationMode = InterpolationMode.HighQualityBicubic;
                        g.SmoothingMode = SmoothingMode.HighQuality;
                        g.PixelOffsetMode = PixelOffsetMode.HighQuality;
                        format = ImageFormat.Jpeg;
                    }
                   
                        g.DrawImage(src,0,0, dst.Width, dst.Height);
                    
                    // At this point the new bitmap has no MimeType
                    // Need to output to memory stream
                    var m = new MemoryStream();
                    dst.Save(m, format);


                    var img = Image.FromStream(m);

                    return img;
                }
            }
        }