If you have rights, you can create a system-level announcement by selecting System Announcements from the menu under your user name and clicking Add a new announcement.
You can create a system announcement that will be displayed for all users on the upper right of all Activity pages, under the Community Announcements heading and above Recommendations. Each announcement will also be displayed as a banner at the top of the Overview Page. You can preview the images and text in your announcement as you create it. If multiple announcements are set to be displayed at the same time, users can click through them in a predetermined sequence.
To create a community announcement and determine the order it will be displayed in: