var bitmap = (Bitmap)Image.FromFile(fullPathToImage);
var hash = ImagePhash.ComputeDigest(bitmap.ToLuminanceImage());

var bitmapSource = BitmapFrame.Create(stream);
var hash = ImagePhash.ComputeDigest(bitmapSource.ToLuminanceImage());

var score = ImagePhash.GetCrossCorrelation(hash1, hash2);

public static (ConcurrentDictionary<string, Digest> filePathsToHashes, ConcurrentDictionary<Digest, HashSet<string>> hashesToFiles) GetHashes(string dirPath, string searchPattern)
{
    var filePathsToHashes = new ConcurrentDictionary<string, Digest>();
    var hashesToFiles = new ConcurrentDictionary<Digest, HashSet<string>>();

    var files = Directory.GetFiles(dirPath, searchPattern);

    Parallel.ForEach(files, (currentFile) =>
    {
        var bitmap = (Bitmap)Image.FromFile(currentFile);
        var hash = ImagePhash.ComputeDigest(bitmap);
        filePathsToHashes[currentFile] = hash;

        HashSet<string> currentFilesForHash;

        lock (hashesToFiles)
        {
            if (!hashesToFiles.TryGetValue(hash, out currentFilesForHash))
            {
                currentFilesForHash = new HashSet<string>();
                hashesToFiles[hash] = currentFilesForHash;
            }
        }

        lock (currentFilesForHash)
        {
            currentFilesForHash.Add(currentFile);
        }
    });

    return (filePathsToHashes, hashesToFiles);
}

(ConcurrentDictionary<string, Digest> gilePathsToHashes, ConcurrentDictionary<Digest, HashSet<string>> hashesToFiles) =
    GetHashes(
        dirPath: @"C:\some\path\",
        searchPattern: "*.jpg");