It's been possible for a while to add "author photos" to posts, but it's a little confusing exactly HOW to do this. In-fact, Squarespace has a helpful page that gets you started in the right direction: journal.squarespace.com/style-by-author ... but they only get you part of the way there. This is because there are so many different ways to configure a blog/journal; EVERY site is different.
If you're a code detective, you can get to the bottom of this mystery, but it's not easy without a little sweaty sleuthing. :)
On a recent client project, I had to re-discover how to do this all over again, as it was about 5 months since I had done it last, so it was a little foggy to me. (see Fruition Partners' blog that I built several months back)
I decided to record a screencast to walk you through this, because it's so much easier to just show you.