Creating thumbnail images with .NET's System.Drawing classes.

ImageManipulation.cs

using System;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;
using System.Linq;

namespace ImageManipulation
{
    /// <summary>
    /// Helper for manipulating images. Resizing images and creating thumbnails.
    /// </summary>
    public static class ImageManipulation
    {
        public static void SaveThumbnail(string imageFilePath, string thumbnailFilePath, int maxWidth, int maxHeight)
        {
            using (Image image = Image.FromFile(imageFilePath))
            {
                if (image.Width <= maxWidth && image.Height <= maxHeight)
                {
                    SaveImage(image, thumbnailFilePath);
                }
                else
                {
                    var ratioX = (double) maxWidth / image.Width;
                    var ratioY = (double) maxHeight / image.Height;
                    var ratio = Math.Min(ratioX, ratioY);
                    var newWidth = (int) (image.Width * ratio);
                    var newHeight = (int) (image.Height * ratio);

                    using (Image thumbnail = Resize(image, newWidth, newHeight))
                    {
                        SaveImage(thumbnail, thumbnailFilePath);
                    }
                }
            }
        }

        public static Image Resize(this Image srcImage, int newWidth, int newHeight)
        {
            Bitmap newImage = new Bitmap(newWidth, newHeight);
            using (Graphics gr = Graphics.FromImage(newImage))
            {
                gr.SmoothingMode = SmoothingMode.HighQuality;
                gr.InterpolationMode = InterpolationMode.HighQualityBicubic;
                gr.PixelOffsetMode = PixelOffsetMode.HighQuality;
                gr.CompositingQuality = CompositingQuality.HighQuality;
                gr.DrawImage(srcImage, new Rectangle(0, 0, newWidth, newHeight));
            }
            return newImage;
        }

        public static void SaveImage(this Image image, string fileName, int quality = 95)
        {
            string fileExtension = fileName.Split('.').Last().Replace("jpg", "jpeg");
            ImageCodecInfo codecInfo = ImageCodecInfo.GetImageEncoders()
                .First(x => x.MimeType.EndsWith(fileExtension, StringComparison.InvariantCultureIgnoreCase));
            var encoderParams = new EncoderParameters
            {
                Param = {[0] = new EncoderParameter(Encoder.Quality, quality)}
            };
            image.Save(fileName, codecInfo, encoderParams);
        }
    }
}