根据ALSeet的URL，获取相册中图片的原图

ALAssetsLibrary * lib = [[ALAssetsLibrary alloc] init];

        [lib assetForURL:assets.assetPropertyURL resultBlock:^(ALAsset *asset){

            //在这里使用asset来获取原图

            ALAssetRepresentation *assetRep = [asset defaultRepresentation];

            CGImageRef imgRef = [assetRep fullResolutionImage];

            UIImage *img = [UIImage imageWithCGImage:imgRef scale:assetRep.scale orientation:(UIImageOrientation)assetRep.orientation];

            //assets.photo = img;//这里是原图

        }

        failureBlock:^(NSError *error){

        }];