I understand the theory. The theory is that by putting your picture on your poster, you make it easier for people to recognize and find you, particularly if the poster for quite a while before the actual poster presentation time. Plus, people are naturally drawn to looking at faces.
I’ve never done this, and I’ve never been a fan of the idea. It always seemed to me to be a little contrived and overly eager. I’m skeptical that a picture works better than a name tag. Posters always have author lists, and attendees have name tags. In my experience, it is rare that there is confusion about whose poster it is.
If you want someone to be able to recognize your face, it can't be the size of a postage stamp on the poster. It would have to be fairly big. Poster real estate is limited and valuable. I am very much in favour of maximizing the work.
Similarly, if you wanted a picture to serve as a way to recognize you, probably the best place to put it is at the top of the poster. Then, you run into a lot of the same layout problems that you have with logos.
If you have collaborators, do you put all the pictures? If not, how will people know which name the photo is associated with?
Finally, not everyone takes a great picture every single time.
Hat tip to the webinar on conference posters, where this question came up.