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()。

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

为什么不用System.Drawing.Image.GetThumbnailImage方法呢?

public Image GetThumbnailImage(
    int thumbWidth, 
    int thumbHeight, 
    Image.GetThumbnailImageAbort callback, 
    IntPtr callbackData)

例子:

Image originalImage = System.Drawing.Image.FromStream(inputStream, true, true);
Image resizedImage = originalImage.GetThumbnailImage(newWidth, (newWidth * originalImage.Height) / originalWidth, null, IntPtr.Zero);
resizedImage.Save(imagePath, ImageFormat.Png);

来源: http://msdn.microsoft.com/en-us/library/system.drawing.image.getthumbnailimage.aspx

本遗嘱-

调整宽度和高度，不需要循环 不超过图像的原始尺寸

//////////////

private void ResizeImage(Image img, double maxWidth, double maxHeight)
{
    double resizeWidth = img.Source.Width;
    double resizeHeight = img.Source.Height;

    double aspect = resizeWidth / resizeHeight;

    if (resizeWidth > maxWidth)
    {
        resizeWidth = maxWidth;
        resizeHeight = resizeWidth / aspect;
    }
    if (resizeHeight > maxHeight)
    {
        aspect = resizeWidth / resizeHeight;
        resizeHeight = maxHeight;
        resizeWidth = resizeHeight * aspect;
    }

    img.Width = resizeWidth;
    img.Height = resizeHeight;
}

注意:这将不适用于ASP。因为WebImage依赖于系统。Web，但在以前版本的ASP。我在Net中多次使用了这个片段，它很有用。

String ThumbfullPath = Path.GetFileNameWithoutExtension(file.FileName) + "80x80.jpg";
var ThumbfullPath2 = Path.Combine(ThumbfullPath, fileThumb);
using (MemoryStream stream = new MemoryStream(System.IO.File.ReadAllBytes(fullPath)))
{
      var thumbnail = new WebImage(stream).Resize(80, 80);
      thumbnail.Save(ThumbfullPath2, "jpg");
}

使用下面的函数和下面的例子来改变图像大小:

//Example : 
System.Net.Mime.MediaTypeNames.Image newImage = System.Net.Mime.MediaTypeNames.Image.FromFile("SampImag.jpg");
System.Net.Mime.MediaTypeNames.Image temImag = FormatImage(newImage, 100, 100);

//image size modification unction   
public static System.Net.Mime.MediaTypeNames.Image FormatImage(System.Net.Mime.MediaTypeNames.Image img, int outputWidth, int outputHeight)
{

    Bitmap outputImage = null;
    Graphics graphics = null;
    try
    {
         outputImage = new Bitmap(outputWidth, outputHeight, System.Drawing.Imaging.PixelFormat.Format16bppRgb555);
         graphics = Graphics.FromImage(outputImage);
         graphics.DrawImage(img, new Rectangle(0, 0, outputWidth, outputHeight),
         new Rectangle(0, 0, img.Width, img.Height), GraphicsUnit.Pixel);

         return outputImage;
     }
     catch (Exception ex)
     {
           return img;
     }
}