After a post is published in P2, you’ll notice that you can edit it inline from the front page – but there doesn’t seem to be an option to add or update an image anymore, like there is in the back end admin interface. Or is there?
Have a look at the “new post box” at the top of the screen. This still has the “Add Media” button, and if you click it you can choose or upload an image, just as if you were writing a new post. All this does is add the embed code for your new image (or media) into the post box. Just copy that and add it to your existing post.
This trick works even if you’re already editing a post.