At the end of 2018, I got an eye-opening finding from my Instagram posts. From 136 posts (image only), my data showed me that the post with natural faces got more likes. In the other hand, the posts that contained merely about ideas (meaning not containing natural faces) got fewer likes. The results can be charted as follows.
Disclaimer: this research is naive to the hour of my post, the increase of my followers, the number of likes I gave to the other accounts near the posting time, image quality, caption length, language, tag, location, topic salience, etc. Also, if the posts contained more than one images, only the first images were counted. It doesn't related to the institution where I work.
The window of time is from January 1st 2018 to December 30 at 12.00 pm.