Assigning categories is really simple, Just click of the square category icon and click on the category you wish to assign. Each time you click on a category, the category will be added in the form of  a small square on the photo. f you assign more then 4 categories, nothing will happen. If you wish to remove a category from a photo, then click on the category square, and click on any highlighted category (Highlighted categories are the categories currently assigned to the selected photo).