我的这个文章是修改简书的blog，本来想在 [作者：郑莫轩 ] 的blog 下评论，但是代码太多了，只能先用一个blog 记录下来，供大家参考。
[代码大放送--iOS图片压缩]
(http://www.jianshu.com/p/974a9537d9f7/comments/2623143#comment-2623143)

- (void)viewDidLoad {
    [super viewDidLoad];
    [self compressedImageFiles:[UIImage imageNamed:@"1.jpg"] imageKB:20 imageBlock:^(UIImage *image) {
        
        
        
    }];
    
    
}

/**
 *  压缩图片
 *
 *  @param image       需要压缩的图片
 *  @param fImageBytes 希望压缩后的大小(以KB为单位)
 *
 *  @return 压缩后的图片
 */
- (void)compressedImageFiles:(UIImage *)image
                     imageKB:(CGFloat)fImageKBytes
                  imageBlock:(void(^)(UIImage *image))block {
    
    __block UIImage *imageCope = image;
    CGFloat fImageBytes = fImageKBytes * 1024;//需要压缩的字节Byte
    
    __block NSData *uploadImageData = nil;
    
    uploadImageData = UIImagePNGRepresentation(imageCope);
    NSLog(@"图片压前缩成 %fKB",uploadImageData.length/1024.0);
    CGSize size = imageCope.size;
    CGFloat imageWidth = size.width;
    CGFloat imageHeight = size.height;
    
    if (uploadImageData.length > fImageBytes && fImageBytes >0) {
        
        dispatch_async(dispatch_queue_create("CompressedImage", DISPATCH_QUEUE_SERIAL), ^{
            
            /* 宽高的比例 **/
            CGFloat ratioOfWH = imageWidth/imageHeight;
            /* 压缩率 **/
            CGFloat compressionRatio = fImageBytes/uploadImageData.length;
            /* 宽度或者高度的压缩率 **/
            CGFloat widthOrHeightCompressionRatio = sqrt(compressionRatio);
            
            CGFloat dWidth   = imageWidth *widthOrHeightCompressionRatio;
            CGFloat dHeight  = imageHeight*widthOrHeightCompressionRatio;
            if (ratioOfWH >0) { /* 宽 > 高,说明宽度的压缩相对来说更大些 **/
                dHeight = dWidth/ratioOfWH;
            }else {
                dWidth  = dHeight*ratioOfWH;
            }
            
            imageCope = [self drawWithWithImage:imageCope width:dWidth height:dHeight];
            uploadImageData = UIImagePNGRepresentation(imageCope);
            
            NSLog(@"当前的图片已经压缩成 %fKB",uploadImageData.length/1024.0);
            //微调
            NSInteger compressCount = 0;
            /* 控制在 1M 以内**/
            while (fabs(uploadImageData.length - fImageBytes) > 1024) {
                /* 再次压缩的比例**/
                CGFloat nextCompressionRatio = 0.9;
                
                if (uploadImageData.length > fImageBytes) {
                    dWidth = dWidth*nextCompressionRatio;
                    dHeight= dHeight*nextCompressionRatio;
                }else {
                    dWidth = dWidth/nextCompressionRatio;
                    dHeight= dHeight/nextCompressionRatio;
                }
                
                imageCope = [self drawWithWithImage:imageCope width:dWidth height:dHeight];
                uploadImageData = UIImagePNGRepresentation(imageCope);
                
                /*防止进入死循环**/
                compressCount ++;
                if (compressCount == 10) {
                    break;
                }
                
            }
            
            NSLog(@"图片已经压缩成 %fKB",uploadImageData.length/1024.0);
            imageCope = [[UIImage alloc] initWithData:uploadImageData];
            
            dispatch_sync(dispatch_get_main_queue(), ^{
                block(imageCope);
            });
        });
    }
    else
    {
        block(imageCope);
    }
}

/* 根据 dWidth dHeight 返回一个新的image**/
- (UIImage *)drawWithWithImage:(UIImage *)imageCope width:(CGFloat)dWidth height:(CGFloat)dHeight{
    
    UIGraphicsBeginImageContext(CGSizeMake(dWidth, dHeight));
    [imageCope drawInRect:CGRectMake(0, 0, dWidth, dHeight)];
    imageCope = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    return imageCope;
    
}