In making a slogan, you need to create your own phrase, sentence or words about a specific topic. For instance, your topic is all about trashes. You can write there, "Tapat ko, linis ko." And then after that, you create your own background. If you want to draw trees, the nature or the environment, then go but the most important is the phrase that you're going to write there. It'll never be called as a slogan if you don't have any words written there; it'll be just called a poster because poster consists of drawing, isn't it?