When you view a photo on another member's profile, or elsewhere on the site, click directly on the photo and you'll see the option to Like it on the right hand side of the photo under the member's name and school.
To Like a photo in a class photo album, click the Like option below each photo. The owner of the photo, as well as any other members who view it, will be able to see how many times the photo has been liked and by whom.
You can also add comments to the photos found in the class photo albums. When you click a photo found in one of the school photo albums, a Comments box will show below each photo and when expanded.
Keep in mind that you can't edit or delete a photo comment or Like after you've clicked Submit. Only the owner of the photo can do so.