It's simple really...
You may or may not want to know this, but it is possible to see every photo you and your friends have ever liked on Facebook. This means that people can find out if you spend your days "liking" inspirational quotes on the pages of actors.
To look at your own history simply type "Photos liked by me" into the search bar.
To see posts by your friends, simply type "Photos like by" followed by your friend's name. This will produce a full list of images they have liked. It's possible to refine the results by year, location and by uploader.
This is just one of the many hidden features within Facebook. This makes it easier for users to look back on archived content on the site.