//图片压
- (void)_imageCompression{
  UIImage *image = [UIImage imageNamed:@"HD"];
  //第一个参数是图片对象，第二个参数是压的系数，其值范围为0~1。
  NSData * imageData = UIImageJPEGRepresentation(image, 0.7);
  UIImage * newImage = [UIImage imageWithData:imageData];
}

/**
 * 图片压缩到指定大小
 * @param targetSize 目标图片的大小
 * @param sourceImage 源图片
 * @return 目标图片
 */
 - (UIImage*)imageByScalingAndCroppingForSize:(CGSize)targetSize withSourceImage:(UIImage *)sourceImage
{
UIImage *newImage = nil;
CGSize imageSize = sourceImage.size;
CGFloat width = imageSize.width;
CGFloat height = imageSize.height;
CGFloat targetWidth = targetSize.width;
CGFloat targetHeight = targetSize.height;
CGFloat scaleFactor = 0.0;
CGFloat scaledWidth = targetWidth;
CGFloat scaledHeight = targetHeight;
CGPoint thumbnailPoint = CGPointMake(0.0,0.0);
if (CGSizeEqualToSize(imageSize, targetSize) == NO)
{
  CGFloat widthFactor = targetWidth / width;
  CGFloat heightFactor = targetHeight / height;
  if (widthFactor > heightFactor)
    scaleFactor = widthFactor; // scale to fit height
  else
    scaleFactor = heightFactor; // scale to fit width
  scaledWidth= width * scaleFactor;
  scaledHeight = height * scaleFactor;
  // center the image
  if (widthFactor > heightFactor)
  {
    thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5;
  }
  else if (widthFactor < heightFactor)
  {
    thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5;
  }
}
UIGraphicsBeginImageContext(targetSize); // this will crop
CGRect thumbnailRect = CGRectZero;
thumbnailRect.origin = thumbnailPoint;
thumbnailRect.size.width= scaledWidth;
thumbnailRect.size.height = scaledHeight;

[sourceImage drawInRect:thumbnailRect];
newImage = UIGraphicsGetImageFromCurrentImageContext();
if(newImage == nil)
  NSLog(@"could not scale image");

//pop the context to get back to the default
UIGraphicsEndImageContext();

 return newImage;
}