private int index = -1; //Keeps track of picture index
        private List<Image> images;
        public Form1()
        {
            InitializeComponent();
            images = new List<Image>();

            DirectoryInfo di = new DirectoryInfo(@"F:\Users\Miroslav\Downloads\pictures");
            FileInfo[] finfox = di.GetFiles("*.jpg", SearchOption.TopDirectoryOnly);
            for (int i = 0; i < finfox.Length; i++)
            {
                FileInfo fi = finfox[i];
                images.Add(Image.FromFile(fi.FullName));
            }
        }

index++;
            if (index < 0 || index >= images.Count)
                index = 0;
            int imageSizeX = images[index].Height;
            int imageSizeY = images[index].Width;
           
            pictureBox1.Height = imageSizeX;
            pictureBox1.Width = imageSizeY;            
            pictureBox1.Image = images[index];