How do I post a photo?
The New Post page has a button labelled "Add Photos" that you can click to add photos when you are submitting new posts. If you have problems adding a photo using the New post page, most groups do allow you to add links to photos in the details section of your post (so you can usually use a link from a 3rd party photo site like Flickr or Dropbox).
Photos can only be added to posts when a post is first submitted (and not after it is published). To learn why photos can't be added later, see the question about editing posts.