Facebook – Keep Facebook likes private

Sharing is the core nature of Facebook and I don't think you can hide it partially, the way you intend to. I don't remember having an option not to show all or some of the likes, or comments, etc. But there is a workaround possible now: with the new version of the privacy settings, you can share only with a subset of friends while hiding your activity from others. But it's all your activity, not just parts of it.

See this question for details of how to exclude some friends from viewing your activity.

Otherwise, just like x3ja said, you would have to stop liking the photos.

The privacy of your like is dependent on the privacy of your friend's post. if they only share the post with you, then you liking it will only appear in your friend's activity feed/newsfeed.

If they share it with a group of friends it will appear only in that group of friends' activity feed/newsfeed.

If they share it publicly. It will end up in all your friend's activity feed/news feed