You add keywords to your images by going to the Content Administration view in your image bank.
Click on "Images" in the left menu, open a collection, and then click on an image to open the side panel where you can add and edit keywords and descriptions.
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.