Can I like a photo, and can I take a like back?

Yes to both. A like is recorded once per guest per photo, and tapping again withdraws it.

Tap the heart on any photo in the gallery. Your like is recorded once — liking the same photo twice does not count twice — and tapping again withdraws it.

Likes are counted against your guest session rather than a name, so they are not shown as coming from you by name, and the host sees totals rather than who liked what.

Likes are not purely decorative: after the photos the host pinned, a photo's like count decides which ones are chosen for the host's highlight reel.