public class BitmapTests
    {
        [Test]
        public void CompareSameTest()
        {
            using (var first = new Bitmap(@"C:\temp\Blank.bmp"))
            {
                using (var other = new Bitmap(@"C:\temp\Blank.bmp"))
                {
                    Assert.IsFalse(first == other);
                }
            }
        }

        [TestCase(@"C:\temp\Blank.bmp", @"C:\temp\Blank.bmp", true)]
        public void CompareBitmapsTest(string firstPath, string otherPath, bool expected)
        {
            using (var first = new Bitmap(firstPath))
            {
                using (var other = new Bitmap(otherPath))
                {
                    Assert.AreEqual(expected, BitmapsEqual(first, other));
                }
            }
        }

        public static bool BitmapsEqual(Bitmap source, Bitmap other)
        {
            if (source == other)  // Compare references
            {
                return true;
            }
            if (source.Size != other.Size)
            {
                throw new ArgumentException("Cannont compare images of different sizes!");
            }
            using (var sourceHelper = new BitMapHelper(source))
            {
                using (var otherHelper = new BitMapHelper(other))
                {
                    return sourceHelper.Bytes.SequenceEqual(otherHelper.Bytes);
                }
            }
        }
    }

    public class BitMapHelper : IDisposable
    {
        private readonly Bitmap _bitmap;
        private BitmapData _bitmapData;
        public byte[] Bytes { get; private set; }
        private int _byteCount;

        public BitMapHelper(Bitmap bitmap)
        {
            _bitmap = bitmap;

            _byteCount = (bitmap.Width + 1)*(bitmap.Height + 1)*Image.GetPixelFormatSize(bitmap.PixelFormat)/8;
            Bytes = new byte[_byteCount];
            _bitmapData = bitmap.LockBits(new Rectangle(0, 0, bitmap.Width, bitmap.Height), ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);
            System.Runtime.InteropServices.Marshal.Copy(_bitmapData.Scan0,Bytes,0,_byteCount);
        }

        public void Dispose()
        {
            _bitmap.UnlockBits(_bitmapData);
        }
    }