从UIImage转换到NSData类型

 //sdk中提供了方法可以直接调用
 UIImage *img = [UIImage imageNamed:@"some.png"];
 NSData *dataObj = UIImageJPEGRepresentation(img, 1.0);

 //下面是sdk中UIImage.h头文件中的内容
 UIKIT_EXTERN NSData *UIImagePNGRepresentation(UIImage *image);                               // return image as PNG. May return nil if image has no CGImageRef or invalid bitmap format
 UIKIT_EXTERN NSData *UIImageJPEGRepresentation(UIImage *image, CGFloat compressionQuality);  // return image as JPEG. May return nil if image has no CGImageRef or invalid bitmap format. compression is 0(most)..1(least)