You can add pictures to a post 2 different ways.
The simplest is as an attachment just like an email.
If you want the pic to be in the actual post (like the pool construction post) you need to upload the photo to a server and get the url for the photo. I have found Imgur to be the easiest to use. You can get a free account and upload your photos. Then select the photo for your post and where it says Share just click on Direct and the url will be copied. Then go to the post you are creating, select the insert image button and paste the pic url in between the 2 img commands.