UIImage图片转成base64字符串：

UIImage *_originImage = [UIImage imageNamed:@"full_playlist_hl.png"];

NSData *_data = UIImageJPEGRepresentation(_image, 1.0f);

NSString *_encodedImageStr = [_data base64Encoding];

NSLog(@"===Encoded image:\n%@", _encodedImageStr);

Base64字符串转UIImage图片：

NSData *_decodedImageData  = [[NSData alloc] initWithBase64Encoding:_encodedImageStr];

UIImage *_decodedImage      = [UIImage imageWithData:_decodedImageData];

NSLog(@"===Decoded image size: %@", NSStringFromCGSize(_decodedImage.size));