There's a specific type of photo that has been getting significantly more engagement on Facebook this year, and it has nothing to do with the subject matter.
Earlier in 2017, Facebook changed the way it displays vertical photos on user's feeds - allowing vertical images to fill significantly more pixel space than they did previously. The result? A far more impactful experience.
Many users are visiting Facebook on devices such as mobile phones and tablets that by nature favor vertical content. With vertical photos now filling the frame of those devices, the possibility of eliciting a reaction from a user is significantly increased. What's more, frame-filling vertical images command more attention, as no other posts can be seen while the image is being viewed.
Consider making a point of utilizing more vertical photos in your posts, and see the results for yourself!