The Media Library
All images and documents uploaded to the website are stored in the Media Library and can be inserted throughout the website on different pages.
You can even insert an image on different pages at different sizes. You can upload a single image and use it as a thumbnail and as a full width image on a page. Drupal will resize it and optimize it for you! Upload the largest image size that you will need.
You can even change an image in the Media Library and it will change all places where it has been inserted. However, the alternate text will not change.