How to get the the dimensions of an image file?

Unfortunately, System.Drawing and ImageProcessor are supported only on the Windows platform because they are a thin wrapper over GDI (Graphics Device Interface) which is very old Win32 (Windows) unmanaged drawing APIs.

Instead, just install the SixLabors/ImageSharp library from NuGet. And here is how you can get size with it:

using (var image = SixLabors.ImageSharp.Image.Load("image-path."))
{
    var width = image.Width;
    var height = image.Height;
}

For .NET Core users and anyone who don't want to use 3rd parity libraries (and like me read the specifications and keep things simple), here is solution for JPEG dimensions:

public class JPEGPicture
{
  private byte[] data;
  private ushort m_width;
  private ushort m_height;

  public byte[] Data { get => data; set => data = value; }
  public ushort Width { get => m_width; set => m_width = value; }
  public ushort Height { get => m_height; set => m_height = value; }
    
  public void GetJPEGSize()
  {
    ushort height = 0;
    ushort width = 0;
    for (int nIndex = 0; nIndex < Data.Length; nIndex++)
    {
      if (Data[nIndex] == 0xFF)
      {
        nIndex++;
        if (nIndex < Data.Length)
        {
          /*
              0xFF, 0xC0,             // SOF0 segement
              0x00, 0x11,             // length of segment depends on the number of components
              0x08,                   // bits per pixel
              0x00, 0x95,             // image height
              0x00, 0xE3,             // image width
              0x03,                   // number of components (should be 1 or 3)
              0x01, 0x22, 0x00,       // 0x01=Y component, 0x22=sampling factor, quantization table number
              0x02, 0x11, 0x01,       // 0x02=Cb component, ...
              0x03, 0x11, 0x01        // 0x03=Cr component, ...
          */
          if (Data[nIndex] == 0xC0)
          {
            Console.WriteLine("0xC0 information:"); // Start Of Frame (baseline DCT)
            nIndex+=4;
            if (nIndex < Data.Length-1)
            {
              // 2 bytes for height
              height = BitConverter.ToUInt16(new byte[2] { Data[++nIndex], Data[nIndex-1] }, 0);
              Console.WriteLine("height = " + height);
            }
            nIndex++;
            if (nIndex < Data.Length - 1)
            {
              // 2 bytes for width
              width = BitConverter.ToUInt16(new byte[2] { Data[++nIndex], Data[nIndex-1] }, 0);
              Console.WriteLine("width = " + width);
            }
          }
        }
      }
    }
    if (height != 0)
      Height = height;
    if (width != 0)
      Width = width;
  }

  public byte[] ImageToByteArray(string ImageName)
  {
    FileStream fs = new FileStream(ImageName, FileMode.Open, FileAccess.Read);
    byte[] ba = new byte[fs.Length];
    fs.Read(ba, 0, Convert.ToInt32(fs.Length));
    fs.Close();

    return ba;
   }
 }

class Program
{
  static void Main(string[] args)
  {
    JPEGPicture pic = new JPEGPicture();
    pic.Data = pic.imageToByteArray("C:\\Test.jpg");
    pic.GetJPEGSize();
  }
}
    

I leave a space for anyone who wants to improve the code for progressive DCT:

if (Data[nIndex] == 0xC2)