任何压缩算法的压缩比都不能无限的提高的。
最有效地方法就是降低图片的分辨率，即像素值。图片的每个像素占用4个字节，假设一般JPG格式的压缩比为1:10的话，40K字节最多只能储存 40000/4*10 = 10万像素的图片，即图片的长乘以宽不超过10万，一般图片长宽比是4:3；所以图片的大小最好不超过 360 X 270 。
降低图片的分辨率就是缩小图片，用Graphics对象、Bitmap对象就能做到。

Bitmap bmpDest = new Bitmap(360,270);
Bitmap bmpSrc = new Bitmap("Source.jpg");
Graphics g = Graphics.FromImge(bmpDest);
g.DrawImage(bmpSrc, rectDest,rectSrc,GraphicsUnit.Pixel); 

或者将PictureBox的长和宽限定在360 X 270

------解决方案--------------------
有压缩算法的。我发给你参考下。
C# code

using System;
using System.Drawing;
using System.Drawing.Imaging;

namespace QQClass
{
    public class CompressImage
    {
        public void Compress_Image(string imageFile,string saveName)
        {
            Bitmap myBitmap;
            ImageCodecInfo myImageCodecInfo;
            Encoder myEncoder;
            EncoderParameter myEncoderParameter;
            EncoderParameters myEncoderParameters;
            // Create a Bitmap object based on a BMP file.
            myBitmap = new Bitmap(imageFile);
            // Get an ImageCodecInfo object that represents the JPEG codec.
            myImageCodecInfo = GetEncoderInfo("image/jpeg");
            // Create an Encoder object based on the GUID
            // for the Quality parameter category.
            myEncoder = Encoder.Quality;
            // Create an EncoderParameters object.
            // An EncoderParameters object has an array of EncoderParameter
            // objects. In this case, there is only one
            // EncoderParameter object in the array.
            myEncoderParameters = new EncoderParameters(1);
            // Save the bitmap as a JPEG file with quality level 25.
            myEncoderParameter = new EncoderParameter(myEncoder, 25L);
            myEncoderParameters.Param[0] = myEncoderParameter;
            try
            {
                myBitmap.Save(saveName, myImageCodecInfo, myEncoderParameters);
            }
            catch
            { 
            
            }

            /*
             Save the bitmap as a JPEG file with quality level 50.
            myEncoderParameter = new EncoderParameter(myEncoder, 50L);
            myEncoderParameters.Param[0] = myEncoderParameter;
            myBitmap.Save("Shapes050.jpg", myImageCodecInfo, myEncoderParameters);
            Save the bitmap as a JPEG file with quality level 75.
            myEncoderParameter = new EncoderParameter(myEncoder, 75L);
            myEncoderParameters.Param[0] = myEncoderParameter;
            myBitmap.Save("Shapes075.jpg", myImageCodecInfo, myEncoderParameters);
            */
        }

        private static ImageCodecInfo GetEncoderInfo(String mimeType)
        {
            int j;
            ImageCodecInfo[] encoders;
            encoders = ImageCodecInfo.GetImageEncoders();
            for (j = 0; j < encoders.Length; ++j)
            {
                if (encoders[j].MimeType == mimeType)
                    return encoders[j];
            }
            return null;
        }
    }
}

------解决方案--------------------
选择jpg格式保存，可缩小的几百K以内，如果在jpg创建时使用了压缩比，则创建的jpg文件可在几十K以内。

------解决方案--------------------
private static ImageCodecInfo GetCodecInfo(string mimeType)
 {
 ImageCodecInfo[] CodecInfo = ImageCodecInfo.GetImageEncoders();
 foreach (ImageCodecInfo ici in CodecInfo)
 {
 if (ici.MimeType == mimeType) return ici;
 }
 return null;
 }

/**//// <summary>
       /// 保存为JPEG格式，支持压缩质量选项
       /// </summary>
       public static bool SaveAsJPEG(Bitmap bmp, string FileName, int Qty)
       {