Thank you for the replies.

I see. All these suggestions would be good if:
a. I hadn't more than a hundred websites to manage (or to give support)
b. The banners weren't be changed all the time
c. The websites weren't managed by people who are simple/end users (clients will manage the websites, which includes older people, etc)

Using skin to manage adding/removing banners would be terrible, as you said (and you mentioned the "skin soup" considering only one site, think about 100+).

Using "display on all pages" and copying modules aren't also a good solution, since the user would have to re-edit all the pages created (or to remove modules from the Admin). Unlike you said, the modules on admin wouldn't be removed only once, because in each new banner added (as I said, they change all the time) this action would be necessary.

I need a built in solution (as you mentioned), because, in fact, the websites will be managed by clients/end users, I will only give support providing the banners (or what is related to the graphic part, content), or solving doubts in managing content, they are independent of me. Well, we are still trying the tool, checking the points it meets our needs and those it doesn't meet.

Rodrigo Moraes wrote:
Using "display on all pages" and copying modules aren't also a good solution, since the user would have to re-edit all the pages created (or to remove modules from the Admin). Unlike you said, the modules on admin wouldn't be removed only once, because in each new banner added (as I said, they change all the time) this action would be necessary.

 Rodrigo,

That's incorrect. To remove a module from All Pages, the page editor simply unchecks the settings Display On All Pages and then only has to delete from a single page.

My suggestion would be to use an Announcements module displayed on all pages, do NOT display the container and customize the announcements item template to only show the body of the announcement.

The end users who add those items can then set start/end dates for individual announcements so that items will disappear automatically.