Admins can add metadata through the Manage page on the web. To add a description or keywords, go to "Images" 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, click on "edit" and start typing a keyword. You separate the keywords either by adding a comma between the words or by clicking enter. Don't forget to click on "Save" once you're done to make sure the keywords stick.
To add keywords to multiple images at the same time, select all images you'd like to tag, press "Add Keywords" and repeat the process from the step above.