First of all you should rename your function, it doesn't perform _just_ a search then `DirSearch()` is not appropriate. What about, for example, `RenameAllPngFiles()`? More context may suggest a better name, _rename_ how? According to which rules? That should be clear.

Parameter names are usually camelCase and they're not prefixed, `sDir` should then be `searchFolder` or even simply `path`.

You do not validate inputs. An I/O error or an invalid argument because calling logic is broken are completely different and they should be handled differently.

    public static void RenameAllPngFiles(string path)
    {
        if (path == null)
            throw new ArgumentNullException(nameof(path));
    
    
        if (String.IsNullOrEmpty(path))
            throw new ArgumentException("Path cannot be empty.", nameof(path));
    
        // ...
    }

We're now at the _core_ of your function. First of all drop `ToList()` because you do not need it.

`GetFiles()` search all files and then you can start processing. If folder is big or you will change to include sub-directories then application will _lag_ until operation is completed, you should experiment using `Path.EnumerateFiles()` which iterates file by file while search proceeds.

100 I/O parallel operations might be too much, you will find a balance only with real-world testing but a reasonable value may be around 4 (especially because each operation is very short and threading overhead may be too high).

Do not catch `Exception` but appropriate exceptions you know may happen: `IOException` and `UnauthorizedAccessException`, anything else is a programming error that should be fixed during development. Also your second catch block is useless because function itself will never throw (you catch and swallow everything). Do not discard the whole batch if one operation fails.

    var images = Directory.GetFiles(path, "*.png", SearchOption.AllDirectories)
        .AsParallel()
        .WithDegreeOfParallelism(MaxParallelIOOperations)
        .ForAll(TryRenameImage);

That's all, note that you can pass function directly without wrapping inside (another) anonymous delegate.

Change your `Rename()` function to handle I/O errors:

    private static void TryRenameImage(string path)
    {
        try
        {
            // Your logic
        }
        catch (IOException e)
        {
            // You may retry few times after a small delay,
            // file might be in use elsewhere.
        }
        catch (UnauthorizedAccessException e)
        {
            // Log?
        }
    }

See also [IOException: The process cannot access the file 'file path' because it is being used by another process][1].

If you write errors to console then you should use `System.Console.Error` instead of `System.Console`. `stdout` is for program output and `stderr` is for its errors.


  [1]: https://stackoverflow.com/q/26741191/1207195