Banners are a new feature in JForum 2.7. Under Admin Control Panel -> Banners you can set up up to 6 banners to be displayed on a page, 3 at the top and 3 at the bottom. Banners can be shown on the board home page, each forum's start page, and on individual topic pages. Under Admin Control Panel -> Configurations it's possible to set on which of these pages -if any- banners should be shown; it can be one or more of "index, forum, topic".
It's possible to have more than one banner in each of the 6 slots, in which case they are rotated acccording to their weight. For example, if there are two banners for "top center", one with a weght of 30, and one with a weight of 60, one will be shown twice as often as the other.
Banners can be active or inactive, so you don't need to delete a banner that you temporarily don't want to show. Three types of banners are available: Text links, image links, and custom HTML code. JForum keeps track of how often banners are shown, and how often they are clicked on (in the case of text and image links).
You could use a single banner at the top center to show a message just on the forum home page (enter just "index" on the Configurations page), and thus show a message to all users only on the forum home page, rather than on all pages - which is what using the "Public announcement" under Configurations would do.