先调整分辨率，分辨率可以自己设定一个值，大于的就缩小到这分辨率，小余的就保持原本分辨率。然后在判断图片数据大小，传入范围maxSize = 100 ，大于的再压缩，小的就保持原样

- (NSData *)compressionImageToDataTargetWH:(CGFloat)targetWH maxFileSize:(NSInteger)maxFileSize  
{  
    if (targetWH <= 0) {  
        targetWH = 1024;  
    }  
    //缩  
    CGSize newSize = CGSizeMake(self.size.width, self.size.height);  
    CGFloat tempHeight = newSize.height / targetWH;  
    CGFloat tempWidth = newSize.width / targetWH;  
    if (tempWidth > 1.0 && tempWidth > tempHeight) {  
        newSize = CGSizeMake(self.size.width / tempWidth, self.size.height / tempWidth);  
    }  
    else if (tempHeight > 1.0 && tempWidth < tempHeight){  
        newSize = CGSizeMake(self.size.width / tempHeight, self.size.height / tempHeight);  
    }  
    UIGraphicsBeginImageContext(newSize);  
    [self drawInRect:CGRectMake(0,0,newSize.width,newSize.height)];  
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();  
    UIGraphicsEndImageContext();  
      
    //压  
    CGFloat compression = 0.9f;  
    CGFloat maxCompression = 0.1f;  
    NSData *imageData = UIImageJPEGRepresentation(newImage, compression);  
    while (imageData.length / 1000 > maxFileSize && compression > maxCompression) {  
        compression -= 0.1;  
        imageData = UIImageJPEGRepresentation(newImage, compression);  
    }  
      
    return imageData;  
}