public string CreateThumbnail(int maxWidth, int maxHeight, string path)
{

    var image = System.Drawing.Image.FromFile(path);
    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);
    var newImage = new Bitmap(newWidth, newHeight);
    Graphics thumbGraph = Graphics.FromImage(newImage);

    thumbGraph.CompositingQuality = CompositingQuality.HighQuality;
    thumbGraph.SmoothingMode = SmoothingMode.HighQuality;
    //thumbGraph.InterpolationMode = InterpolationMode.HighQualityBicubic;

    thumbGraph.DrawImage(image, 0, 0, newWidth, newHeight);
    image.Dispose();

    string fileRelativePath = "newsizeimages/" + maxWidth + Path.GetFileName(path);
    newImage.Save(Server.MapPath(fileRelativePath), newImage.RawFormat);
    return fileRelativePath;
}

点击这里http://bhupendrasinghsaini.blogspot.in/2014/07/resize-image-in-c.html

调整大小并保存图像以适应宽度和高度，就像画布保持图像成比例

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

namespace Infra.Files
{
    public static class GenerateThumb
    {
        /// <summary>
        /// Resize and save an image to fit under width and height like a canvas keeping things proportional
        /// </summary>
        /// <param name="originalImagePath"></param>
        /// <param name="thumbImagePath"></param>
        /// <param name="newWidth"></param>
        /// <param name="newHeight"></param>
        public static void GenerateThumbImage(string originalImagePath, string thumbImagePath, int newWidth, int newHeight)
        {
            Bitmap srcBmp = new Bitmap(originalImagePath);
            float ratio = 1;
            float minSize = Math.Min(newHeight, newHeight);

            if (srcBmp.Width > srcBmp.Height)
            {
                ratio = minSize / (float)srcBmp.Width;
            }
            else
            {
                ratio = minSize / (float)srcBmp.Height;
            }

            SizeF newSize = new SizeF(srcBmp.Width * ratio, srcBmp.Height * ratio);
            Bitmap target = new Bitmap((int)newSize.Width, (int)newSize.Height);

            using (Graphics graphics = Graphics.FromImage(target))
            {
                graphics.CompositingQuality = CompositingQuality.HighSpeed;
                graphics.InterpolationMode = InterpolationMode.HighQualityBicubic;
                graphics.CompositingMode = CompositingMode.SourceCopy;
                graphics.DrawImage(srcBmp, 0, 0, newSize.Width, newSize.Height);

                using (MemoryStream memoryStream = new MemoryStream())
                {
                    target.Save(thumbImagePath);
                }
            }
        }
    }
}

不知道这有什么困难，做你正在做的，使用重载Bitmap构造函数来创建一个重新大小的图像，你唯一缺少的是转换回image数据类型:

public static Image resizeImage(Image imgToResize, Size size)
{
    return (Image)(new Bitmap(imgToResize, size));
}

yourImage = resizeImage(yourImage, new Size(50,50));

如果你正在使用BitmapSource:

var resizedBitmap = new TransformedBitmap(
    bitmapSource,
    new ScaleTransform(scaleX, scaleY));

如果你想要更好的控制质量，首先运行这个:

RenderOptions.SetBitmapScalingMode(
    bitmapSource,
    BitmapScalingMode.HighQuality);

(默认为BitmapScalingMode。线性，相当于BitmapScalingMode.LowQuality。)

你可以试试net-vips, libvips的c#绑定。它是一个懒惰的、流的、需求驱动的图像处理库，所以它可以做这样的操作，而不需要加载整个图像。

例如，它附带了一个方便的图像缩略图:

using Image image = Image.Thumbnail("image.jpg", 300, 300);
image.WriteToFile("my-thumbnail.jpg");

它还支持智能裁剪，这是一种智能确定图像中最重要的部分并在裁剪图像时保持对焦的方法。例如:

using Image image = Image.Thumbnail("owl.jpg", 128, crop: Enums.Interesting.Attention);
image.WriteToFile("tn_owl.jpg");

其中owl.jpg是一个偏离中心的合成:

给出如下结果:

首先，它将图像的垂直轴缩小到128像素，然后使用注意力策略将图像缩小到128像素。这一个搜索图像中可能吸引人眼的特征，详见Smartcrop()。

在我做的应用程序中，有必要创建一个具有多个选项的函数。它相当大，但它可以调整图像的大小，可以保持纵横比，并可以切割边缘，只返回图像的中心:

/// <summary>
    /// Resize image with a directory as source
    /// </summary>
    /// <param name="OriginalFileLocation">Image location</param>
    /// <param name="heigth">new height</param>
    /// <param name="width">new width</param>
    /// <param name="keepAspectRatio">keep the aspect ratio</param>
    /// <param name="getCenter">return the center bit of the image</param>
    /// <returns>image with new dimentions</returns>
    public Image resizeImageFromFile(String OriginalFileLocation, int heigth, int width, Boolean keepAspectRatio, Boolean getCenter)
    {
        int newheigth = heigth;
        System.Drawing.Image FullsizeImage = System.Drawing.Image.FromFile(OriginalFileLocation);

        // Prevent using images internal thumbnail
        FullsizeImage.RotateFlip(System.Drawing.RotateFlipType.Rotate180FlipNone);
        FullsizeImage.RotateFlip(System.Drawing.RotateFlipType.Rotate180FlipNone);

        if (keepAspectRatio || getCenter)
        {
            int bmpY = 0;
            double resize = (double)FullsizeImage.Width / (double)width;//get the resize vector
            if (getCenter)
            {
                bmpY = (int)((FullsizeImage.Height - (heigth * resize)) / 2);// gives the Y value of the part that will be cut off, to show only the part in the center
                Rectangle section = new Rectangle(new Point(0, bmpY), new Size(FullsizeImage.Width, (int)(heigth * resize)));// create the section to cut of the original image
                //System.Console.WriteLine("the section that will be cut off: " + section.Size.ToString() + " the Y value is minimized by: " + bmpY);
                Bitmap orImg = new Bitmap((Bitmap)FullsizeImage);//for the correct effect convert image to bitmap.
                FullsizeImage.Dispose();//clear the original image
                using (Bitmap tempImg = new Bitmap(section.Width, section.Height))
                {
                    Graphics cutImg = Graphics.FromImage(tempImg);//              set the file to save the new image to.
                    cutImg.DrawImage(orImg, 0, 0, section, GraphicsUnit.Pixel);// cut the image and save it to tempImg
                    FullsizeImage = tempImg;//save the tempImg as FullsizeImage for resizing later
                    orImg.Dispose();
                    cutImg.Dispose();
                    return FullsizeImage.GetThumbnailImage(width, heigth, null, IntPtr.Zero);
                }
            }
            else newheigth = (int)(FullsizeImage.Height / resize);//  set the new heigth of the current image
        }//return the image resized to the given heigth and width
        return FullsizeImage.GetThumbnailImage(width, newheigth, null, IntPtr.Zero);
    }

为了更容易访问函数，可以添加一些重载函数:

/// <summary>
    /// Resize image with a directory as source
    /// </summary>
    /// <param name="OriginalFileLocation">Image location</param>
    /// <param name="heigth">new height</param>
    /// <param name="width">new width</param>
    /// <returns>image with new dimentions</returns>
    public Image resizeImageFromFile(String OriginalFileLocation, int heigth, int width)
    {
        return resizeImageFromFile(OriginalFileLocation, heigth, width, false, false);
    }

    /// <summary>
    /// Resize image with a directory as source
    /// </summary>
    /// <param name="OriginalFileLocation">Image location</param>
    /// <param name="heigth">new height</param>
    /// <param name="width">new width</param>
    /// <param name="keepAspectRatio">keep the aspect ratio</param>
    /// <returns>image with new dimentions</returns>
    public Image resizeImageFromFile(String OriginalFileLocation, int heigth, int width, Boolean keepAspectRatio)
    {
        return resizeImageFromFile(OriginalFileLocation, heigth, width, keepAspectRatio, false);
    }

现在是最后两个可选的布尔值。 像这样调用函数:

System.Drawing.Image ResizedImage = resizeImageFromFile(imageLocation, 800, 400, true, true);