Simple utility that takes image (where width equals to height) and make isometric tile out of it.

IsoTexture.cs

using System;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;
using System.IO;

namespace Barbar.IsoTexture
{
    class Program {
    static int Main(string[] args) {
      if (args == null || args.Length != 2 || string.IsNullOrEmpty(args[0]) || string.IsNullOrEmpty(args[1])) {
        Console.Out.WriteLine("Usage:");
        Console.Out.WriteLine("isotexture [input.(gif|png|jpg)] [output]");
        return -1;
      }

      string basePath = AppDomain.CurrentDomain.BaseDirectory;
      string input, output;
      if (Path.IsPathRooted(args[0])) {
        input = args[0];
      } else {
        input = Path.Combine(basePath, args[0]);
      }

      if (Path.IsPathRooted(args[1])) {
        output = args[1] + ".png";
      } else {
        output = Path.Combine(basePath, args[1] + ".png");
      }

      if (!File.Exists(input)) {
        Console.Error.WriteLine("Input file does not exists.");
        return -1;
      }

      if (File.Exists(output)) {
        Console.Out.WriteLine("Output file exists, overwrite ? [Y/N]");
        do {
          string response = Console.In.ReadLine();
          if (response.StartsWith("Y", StringComparison.OrdinalIgnoreCase)) {
            break;
          }
          if (response.StartsWith("N", StringComparison.OrdinalIgnoreCase)) {
            return -1;
          }
        } while (true);
      }

      Bitmap inputImage = new Bitmap(input);
      if (inputImage.Width != inputImage.Height) {
        Console.Error.WriteLine("Image width must match image height.");
        return -1;
      }

      var outputImage = CreateRotatedTexture(inputImage, 18);
      outputImage.Save(output, ImageFormat.Png);

      return 0;
    }

    private static Bitmap CreateRotatedTexture(Bitmap inputImage, int slices) {
      if (inputImage == null)
        throw new ArgumentNullException("inputImage");

      if (inputImage.Width != inputImage.Height)
        throw new ArgumentException("Width and height of image must match.", "inputImage");

      double xscale = Math.Pow(3, 0.5);

      int size = inputImage.Width;
      var outputImage = new Bitmap((int)(xscale * size), size * slices, PixelFormat.Format32bppPArgb);

      using (Graphics graphics = Graphics.FromImage(outputImage)) {
        graphics.FillRectangle(new SolidBrush(Color.Transparent), new Rectangle(0, 0, outputImage.Width, outputImage.Height));
      }

      for (var i = 0; i < slices; i++) {
        using (Graphics graphics = Graphics.FromImage(outputImage)) {
          Matrix matrix = new Matrix();
          matrix.Translate(0, i * size);
          matrix.Translate((float)(xscale * 0.5 * (double)size), size / 2);
          matrix.Scale((float)Math.Pow(1.5, 0.5), (float)Math.Pow(2, -0.5));
          matrix.Rotate(-45 + (360 / slices) * i);
          matrix.Translate(-size / 2, -size / 2);

          graphics.Transform = matrix;
          graphics.DrawImage(inputImage, new Point(0, 0));
          graphics.Flush();
        }
      }

      return outputImage;
    }


    private static Bitmap CreateIsoTexture(Bitmap inputImage) {
      if (inputImage == null)
        throw new ArgumentNullException("inputImage");

      if (inputImage.Width != inputImage.Height)
        throw new ArgumentException("Width and height of image must match.", "inputImage");

      double xscale = Math.Pow(3, 0.5);

      int size = inputImage.Width;
      Bitmap outputImage = new Bitmap((int)(xscale * size), size, PixelFormat.Format32bppPArgb);

      Matrix matrix = new Matrix();
      matrix.Translate((float)(xscale * 0.5 * (double)size), size / 2);
      matrix.Scale((float)Math.Pow(1.5, 0.5), (float)Math.Pow(2, -0.5));
      matrix.Rotate(-45);
      matrix.Translate(-size / 2, -size / 2);

      using (Graphics graphics = Graphics.FromImage(outputImage)) {
        graphics.FillRectangle(new SolidBrush(Color.Transparent), new Rectangle(0, 0, outputImage.Width, outputImage.Height));
        graphics.Transform = matrix;
        var attributes = new ImageAttributes();
        //attributes.SetColorKey(inputImage.GetPixel(0, 0), inputImage.GetPixel(0, 0));

        graphics.DrawImage(inputImage, new Point(0,0));
      }

      return outputImage;
    }
  }
}