This ‘Adding Media Library Images’ tutorial covers:
The ‘Media Library’ stores all of the images uploaded to your site in one place–making it easy to access images in the future. To add an image to a post or a page that is already in the ‘Media Library’ place the cursor where you want image to display and click the ‘Add Media’ button.
The ‘Insert Media’ screen will appear giving you the option to ‘Upload Files’ or click the tab that says ‘Media Library’ and all of your previously loaded media files will appear. Simply select the image you would like to insert into the post and options for editing the image will show to the right of the Media Library.