AVAssetImageGenerator *imageGenerator = [[AVAssetImageGenerator alloc]initWithAsset:asset];
CGImageRef stillImageRef = [imageGenerator copyCGImageAtTime:CMTimeMakeWithSeconds(CMTimeGetSeconds(asset.duration), asset.duration.timescale) actualTime:NULL error:NULL];
UIImage *stillImage = [UIImage imageWithCGImage:stillImageRef];

- (void)captureOutput:(AVCaptureFileOutput *)captureOutput didFinishRecordingToOutputFileAtURL:(NSURL *)outputFileURL fromConnections:(NSArray *)connections error:(NSError *)error {

    AVAsset *asset = [AVAsset assetWithURL:outputFileURL];
    ...