Once you login to the WordPress administration page, you will see a sidebar menu. Then on the sidebar menu, Hover on Media, click on Add New.
You will be directed to the Upload New Media page. In that page, you can upload your image file(s) by drag and drop method or by clicking Select files button. After you dropped or selected the image, the image will automatically upload to the library. In web design perpective, we recommend you save as web using a photo editing software before you upload to the CMS Media Library.