If you already have in your media library that you would like to add to your post, simply click “add media” in your editor. WordPress gives you the option to add your own media, but if you click the next tab you will have access to all your previously uploaded content in your media library.
Just select the image you want, adjust the settings how you like, and click “insert into post”. Your image will automatically be added to wherever your cursor was in the post. You can then click preview to see how your post will look, or hit publish to make your post public.