UIImage *image = [[UIImage alloc] initWithData:data];//大图
CGRect imageRect = CGRectMake(0.0, 0.0, image.size.width, image.size.height);
CGSize itemSize = CGSizeMake(w,h);
UIGraphicsBeginImageContext(itemSize);
[image drawInRect:imageRect];
UIImage *smallimage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();