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.
Where are the post photos?
Usually the photos are linked to or shown with the post when you view a post on the site or in the app. But some people access the groups from the Yahoo Groups site which has a separate photo section. Posts with photos from Yahoo Groups will often have a dropdown menu below the post details. The dropdown menu should allow you to open and view the photo albums from Yahoo Groups.
Sadly, due to limitations in how Yahoo Groups works, the specific photo or album referenced in the post may not always be available. If this happens, it's usually easiest to contact the post author directly and ask them to send you the photos. The only other alternative is access the photo section directly on the Yahoo Groups site (if you aren't a member of the group on Yahoo Groups, you'll have to join the group on Yahoo Groups to access the photos - see links to Yahoo Groups).