Any user with Site Owner or Site Administrator privileges is able to edit the footer section of their site. Content added here will be visible on every page of your site.
If you scroll to the bottom of your page while logged in, you should see three "Edit" buttons across the page:
Clicking on any one of these buttons will open a dialog box containing a text editor field. This field will allow you to add text and images in the same manner as any Page or content item that uses the same editor. Once you are done making your changes, scroll down to the Save button below the text box.
The three Edit buttons represent three columns that are available in the footer. These allow you to place different content across the width of your site footer; however there is no way to combine them to create a single consolidated footer.
Note that there is currently an issue where sometimes, when you click on the Edit button, it will take you to a page with a small text box instead of the text editor dialog. If this happens, use the back button on your browser to return to your main page, and then force a refresh of your page (Ctrl+F5 will work in Chrome, Firefox, and Internet Explorer).