If you already have an image in your media library that you would like to put into your post, simply click Add Media in your editor. WordPress gives you the option to upload your own media.
But if click over to the next tab, you 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. Hit Preview to see what your post will look like or hit Publish to make your post public.