How do I see who liked my picture or someone else’s?

There are several ways to know who likes your post. You can see the most recent activity on the activity board. (Make sure to check the blog entry named How do I see the activity related to my profile (comments, friend requests, likes)?). 

You can also check who liked the photo (either yours or someone else's post) by clicking on the number next to the heart button at the bottom left-hand corner.