Adding Images Through Pages/Posts
When writing or editing a post, you can add an image using the Add Media button above the editor box. This opens a window that displays the multi-file uploader. From here you can drag and drop your picture or select it using the file selector.
The images you add here will be automatically added to the Media Library.
Adding Images Directly To The Media Library
If you want to upload an image for a future post, from the dashboard go to Media --> Add New. From here you can bulk upload multiple images before you know which post you want them to go into.
Adding Images By FTP
If you know what an FTP program is, you can upload images directly to the /wp-content/uploads folder.
For more information, see FTP Client