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!
As a medium, photos are by far the most engaged-with content on social media. The entire founding of the platforms of Instagram and Snapchat speak to that. Facebook itself encourages administrators of pages to post more posts with photos. It's said that a photo speaks a thousand words, in part, because people see and process them with an immediacy not possible with text or video. You instantly have a feeling when seeing a photo.
There's a rarely spoken-of key to helping drive even more successful engagement with your photos, and it's almost counter-intuitive at first glance. Whenever you post a photo on social media that you're hoping to get a good response from - take a moment to see if the photo looks good small, seriously. Many people take the time to make sure their photos looks good large - and all the fine details are exactly as they want - and you absolutely should, but if a photo can't be understood on a smaller scale, those details may never be seen. If it's not worth viewing, it's a lot less likely to be reacted to, commented on, or shared, and may end up being passed up entirely.
Consider that many of your followers are viewing social media on a small device, with a smaller screen, likely in the midst of doing three other things. If a photo doesn't look intriguing enough to be understood at 400 pixels (or sometimes far less), you may be missing out on engagement that you could've had with a more appealing photo.
Before you post, take a look at the thumbnail of the photo and see if you'd want to view it larger.