Thank you for the question.
As we can see on your web-site you have already added categories element to your web-site. So all you need to do now is to upload your images for all categories. Note that all should be squared except one (it should be twice larger than others, like on our demo). You can try to upload our image for this category for testing purposes http://demo.xtemos.com/basel/wp-content/uploads/2015/08/cat-watch-2-379×800.jpg