In the business settings you can find the promotional banners under the section "Marketing".
Here you can add promotional banners that are displayed on the community page and the mobile app. You can for example promote your website, your Facebook page or a certain discount you're offering. Just upload a picture, put up a link and fill in a text. This will be displayed on top of the picture. You can even choose a certain time limit when the promo banner is shown. So if a discount only lasts a month, you can fill that in and it will automatically be disabled after a month.