Admins can add metadata through the Manage page on the web. To add a description or keywords, go to "Your Library" on the Manage page on the Web and then navigate to the image you'd like to add the metadata to.
To add a description, click on the image and then press "Edit" next to "Description" on the right-side panel that opens up. Write your description, then press "Save".
To add keywords, press "Add Keywords" on the right-side panel and start writing the keywords. Don't forget to press Enter after every keyword. When you're done adding keywords, press Save.
To add keywords to multiple images at the same time, select all images you'd like to tag, press "Add Keywords" next to the "Add common keywords" option and repeat the process from the step above.