Select the Resources tool from the Tool Menu of your site.
To the right of the folder to which you want to add files, click the Actions drop-down menu and select Upload Files.
If you have not created any folders yet, you can use the site's root folder (bearing the name of the site) to add files, or create them by following the steps here.
Drag files from your computer and drop them in the box labeled Drop files to upload, or click here to browse.
This will display thumbnails of the files that will be uploaded.